DE19741122C2 - Arrangement for measurement and structuring (near field arrangement) - Google Patents

Description

The invention relates to an arrangement according to the preamble of Claim 1.

So that optical arrangements that at a radiation exit opening exploit existing near field, e.g. B. near-field optical microscope Being able to measure or structure samples, the distance to the sample must be checked when scanning.

It is known to use optical near-field arrangements for measuring or for writing / reading. Such an arrangement, which uses the intensity effects in a fiber laser to determine the distance, is described by D. Betzig and others in Appl. Phys. Lett. 63 (26), 1993, page 3550 | 1 | described. The use of a near field arrangement for storing or reading out data is described by E. Betzig inter alia in Appl. Phys. Lett. 61 (2), 1992, page 142. In addition, 32 patents are listed with the corresponding patent numbers (US Patent Database), which reflect the device and application aspect:
5214282: Method and apparatus for processing a minute portion of a specimen
5539197: Scanning near-field optical microscope having a medium denser than air in the gap between the probe chip and the sample being measured
5333495: Method and apparatus for processing a minute portion of a specimen
5479024: Method and apparatus for performing near-field optical microscopy
5581083: Method for fabricating a sensor on a probe tip used for atomic force microscopy and the like
5517280: Photolithography system
5294790: Probe unit for near-field optical scanning microscope
5473157: Variable temperature near-field optical microscope
5449901: Fine surface observing apparatus
5354985: Near field scanning optical and force microscope including cantilever and optical waveguide
5018865: Photon scanning tunneling microscopy
4829177: Point projection photoelectron microscope with hollow needle
5583286: Integrated sensor for scanning probe microscope
5581193: Multiple source and detection frequencies in detecting threshold phenomena associated with and / or atomic or molecular spectra
5559328: Small cavity analytical instruments
5548113: Co-axial detection and illumination with shear force dithering in a near-field scanning optical microscope
5546223: Method for external excitation of subwavelength light sources that is integrated into feedback methodologies
5538898: Method suitable for identifying a code sequence of a biomolecule
5504366: System for analyzing surfaces of samples
5471064: Precision machining method, precision machining apparatus and data storage, apparatus using the same
5469734: Scanning apparatus linearization and calibration system
5461600: Hig-density optical data storage unit and method for writing and reading information
5397896: Multiple source and detection frequencies in detecting threshold phenomena associated with and / or atomic or molecular spectra
5382789: Near field scanning optical microscope
5304795: High resolution observation apparatus with photon scanning microscope
5288999: Manufacturing method including near-field optical microscopic examination of a semiconductor wafer
5288998: Manufacturing method including photoresist processing using a near-field optical probe
5288997: Manufacturing method, including near-field optical microscopic examination of a magnetic bit pattern
5288996: Near field optical microscopic examination of genetic material
5281814: System for imaging and detecting threshold phenomena associated with and / or atomic or molecular spectra of a substance by reflection of an AC electrical signal
5199090: Flying magnetooptical read / write head employing an optical integrated circuit waveguide
5105305: Near field scanning optical microscope using a fluorescent probe

Interferometric methods for determining the distance between Radiation outlet and sample are from S. Pilevar u. a. in Ultramicroscopy 61, 1995, page 223 | 2 | or by U. Schwarz u. a. in opt. Comm. 134, 1997, page 301 | 3 | described.

The known methods for determining the distance are based on a direct one Interaction of fiber tip and sample. Checking the distance fiber tip Trials can be accomplished in different ways.

The most common control mechanism is based on measuring the Damping an oscillating tip due to shear forces (E. Betzig et al. In Appl. Phys. Lett. 60, 1992, page 2484). This attenuation is measured optically in that a laser beam is directed laterally onto the tip and the modulation of the laser intensity caused by the movement of the tip is measured. There are a variety of variations on this process.  

The following serious disadvantages cannot be eliminated. The movement of the tip must be monitored at a very short distance from the sample (<100 nm); sample roughness means that the laser intensity varies without there being any damping of the sample movement. The laser intensity irradiated from the side is approx. 10 ⁵ times more intense than the light that passes through the fiber tip and, if the sample is examined spectroscopically, can lead to falsification of the measurement result via scattered radiation. In addition, the control system must be set up each time the tip is replaced.

Another distance control method is based on the use of Tunnel microscopy (U. Dürig et al. In J. Appl. Phys. 59, 1986, page 3318). For The method is very suitable for homogeneous samples. The main disadvantage is that the samples must be conductive. Other methods are based on the Use of atomic force microscopes (AFM) described by N.F. Hulst u. a. in scan sample microsc. 36, 1992, page 36. An interesting one Control mechanism for the tip-sample distance was developed by K. Karrai et al. a. in Appl. Phys. Lett. 66, 1995, page 1842, the fiber tip attached to an arm of a quartz tuning fork. By interaction of the At the tip of the sample there is a damping of the measuring system, which is reflected in a Changes in the resonance circuit quality expresses. The high has a disadvantage Preparation effort. The high circular quality means that the scanning (- scan) speed is low. An attempt was further made to Change in impedance of a shear force damped piezo oscillator Use distance control (J. W. Hsu et al. In Rev. Sci. Instr. 66, 1995, Page 3177). Typical control frequencies (scan rates) of the methods mentioned are less than or equal to 10 kHz.

With the methods mentioned, a safety margin must be observed cannot be guaranteed between the radiation exit opening and the sample. The Betzig | 1 | suggests using a fiber laser in which one of the Fiber ends is extended to a tip and at which the sample as  End reflector used for the laser (3-mirror laser), allowed Changes in reflection on the sample surface in changes in intensity implement. The distance control is carried out according to the tried and tested Standard methods.

The in | 2 | described method is limited to the metrological aspect of signal processing, whereby that of the sample in the radiation entrance opening reflects back light with a reference beam is compared (Mach-Zehnder interferometer). The in | 3 | described The arrangement is the same as that in | 1 | described measuring method, with Except for the lasers used.

The change in laser intensity, which is caused by the change in reflection on the sample surface is caused by changing the quality of the resonator coupled to the laser. 3 Mirror arrangements, of which in | 1 | and | 3 | are already reported by Eichler u. a. in journal f. Applied Physics 28 (3), 1969, page 125 | 4 | and heartfelt u. a. in Phys. Lett 24A (12) 1967, page 684.

The in | 1 | - | 3 | The methods described are based on the separation of the methods for recording the measurement signal in sample spectroscopy and control to maintain the distance sample-fiber tip in case the sample to be measured in a spatially resolved manner. The former is purely optical, the latter uses it techniques described above, which is essentially mechanical in nature. Furthermore, they are in | 1 | described procedures and the corresponding Follow-up procedures by those occurring in the pulse operation of solid-state lasers Relaxation oscillation limited in the scanning speed.

A device for exact positioning of a laser to one measuring surface is described in DE 195 31 802. Here are symmetrical to the pointed end of a fiber laser Collection fibers arranged, their imaginary axes the tip of the  Cutting fiber lasers. Different optical fibers are attached to the collection fibers Devices switched on, the outputs of which are connected to a computer, which depending on the received output signals Controls positioning platform with the surface to be measured.

Another interferometric near-field arrangement is in EP 0 757 271 A2 described in which an electromagnetic radiation source leading wave generated, received by a probe tip and as Signal wave is sent back. This signal wave is with a Reference wave superimposed and the interference signal is through a detector evaluated. This device has means for positioning the Probe tip and the workpiece so close together that a multi-pole Coupling effect arises, whereby the stray electric field strength Signal wave is increased.

The main disadvantage of all the methods mentioned, the process-specific in sample spectroscopy with low Light intensity must be based on the measuring principle, that is based solely on the detection of changes in intensity, so that the signal Noise ratio cannot meet the requirements. The technological Effort z. B. in | 2 | is driven to remedy this disadvantage disproportionate to the results achieved.

The object of the invention is an arrangement for material characterization as well as for reading or writing information in the nanometer range to create the sample distance control and the working process based on The sample is standardized, which is compact, easy to use and inexpensive is.

This object is achieved by an arrangement with the features of claim 1 solved.  

Diode-pumped fiber lasers in combination with fiber amplifiers offer one promising starting point for the realization more compact and easy-to-use interferometric near-field arrangements based on the Principle of a laser interferometer based. Near field arrangements of this type benefit from the development of fiber lasers, whose emission spectrum of From ultraviolet to infrared, and from the perfection of technology the manufacture of fiber tips. In addition to the emission wavelength of The form of emission - constant light, pulse - can also be selected with fiber laser.

The interferometric near-field arrangement mentioned advantageously combines A fiber laser with an undamped measuring resonator.

In an advantageous embodiment, the measuring resonator contains a radiation-amplifying medium, which is also designed as a fiber. The Fiber lasers and the fiber amplifier are made of the same rare earth ions endowed in a known manner. The level of the doping concentration required for the Laser or amplifier effect is decisive, can in a known manner can be chosen differently. The fiber that is intended as an amplifier (Fiber amplifier), is unilaterally by known methods, with a tip Mistake. The diameter of the radiation exit opening of the tip can be in be adapted to the requirements in a known manner. That too investigating material (sample), as a third mirror, completed in known See the laser interferometer, see | 4 |. A major advantage of Interferometric near-field arrangement mentioned is that the measurement signal a laser frequency change associated with a change in distance is used. The calibration of the frequency change with a known change in distance allows the absolute specification of a sample height profile. The output power of the laser, as in the described arrangements | 1 |, | 3 |, stands as another Measured variable available as it is used for local refractive, reflective or absorptive changes on the sample also varied. The superiority of mentioned arrangement with respect to the signal-to-noise ratio is one  To use comparison, the transition from AM radio reception to FM Analog reception.

Settling processes (relaxation oscillations) play in the above Arrangement does not matter because the laser works in a stationary state. The Radiation losses at the tapered fiber end are in the Arrangement advantageously through the reinforcement in the fiber amplifier compensated. Another advantage of the arrangement for spectroscopic applications exist in the range of a few nanometers Wavelength tunability of the laser.

According to the invention, with the arrangement mentioned, height differences .DELTA.L _V on a sample surface can be detected in the range 10 ^-2 nm <.DELTA.L _v <100 nm. The particular advantage of the interferometric near-field arrangement described here lies in the detection of small changes in refractive index or degree of absorption, which takes place via intensity feedback. With a spatial resolution of <100 nm, differences in reflectance <10 ^-6 , refractive index differences Δn of Δn <10 ^-5 and absorption degree variations of approx. 10 ^-3 can be detected. According to the invention, the laser length can be shifted by at least λ / 2.

For the data storage area of application, the near field arrangement described can be used to process data of very high packing density (writing, reading). Target values are e.g. B. storage densities greater than 200 bit / µm ² at a write speed of about 10 Mbit / s or a read speed of 1 to 10 Mbit / s.

The invention is in the field of laser measurement technology, in particular in Areas in which the spatial resolution is in the nanometer range. The technical problem with which the invention is concerned is one To create an arrangement that allows the distance over a contactless  Keep sample constant in the nanometer range and local changes absorptive, refractive and reflexive nature, which a spatial extension in the Possess nanometer range. According to the invention time-resolved measurements possible. The arrangement is said to be compact, be user-friendly and insensitive to environmental influences. The Control speed (scan rate) should be greater than in known methods and an increase in sensitivity when detecting reflective, refractive and Enable absorptive changes (sample spectroscopy).

For this purpose, the solution according to the invention provides that a diode-pumped Fiber laser and a fiber amplifier pumped with the same diode in one 3 mirror arrangement are summarized. The fiber laser is a 2- Mode laser trained as he z. B. by I. M. Jauncay u. a. Electr. Lett. 24 (1), 1988, page 24 | 5 | is described. The fiber amplifier is reflection-free at the Spliced fiber laser. The third mirror is realized by the sample that thereby being an integral part and object of investigation at the same time. The The reaction of the sample to the laser properties can be formalized coupled resonators | 4 | describe.

The fiber laser and fiber amplifier use to generate radiation or amplification one doped standard monomode fiber. The fiber core is a transversely monomodal optical fiber that is cladding is surrounded.

The diode laser radiation, whose beam parameter product matches that of the laser fiber should be adapted, the radiation-amplifying material is used, both in the core of the laser and in the amplifier fiber is stored to pump. Optical pumping takes place according to the invention according to the tandem principle, d. H. the in the radiation-enhancing material of the Laser's non-absorbed portion of the pump radiation is used to pump the radiation-amplifying material used in the amplifier. The fiber that the Fiber amplifier forms, is tapered and unilaterally by known methods has a light exit opening at the tip, the diameter of which according to  Manufacturing technology in the 100 nm range (or below). The coupled resonator (external resonator) is by a mirror, the Laser resonator and external resonator have in common (center mirror), and the sample opposite the tip was formed. The invention The principle of operation of the arrangement can be described as follows. The determination the height profile when scanning the sample is laser interferometric realized. For this it is necessary to determine the intensity emitted by the laser Undergo frequency analysis. According to the invention Near field arrangement exploited the change in a laser frequency, which at Length change of the external resonator occurs. The difference frequency two laser modes, one of which is said to be unaffected, is called Measurement signal used. The different lengths of the external resonator, which are determined by the height profile of the sample (nm range) directly the measurable difference frequencies. The above-mentioned rule principle for Determining the height profile of the sample takes advantage of that Resonance frequency of the laser (mode) is shifted when a Resonance frequency (mode) of the external resonator is sufficiently tight is adjacent. Because the length variation of the external resonator accordingly the height variation on the sample is in the nm range, is always close a laser resonance worked. In the case of coincidence the influencing mode with a mode of the external resonator, one measures the undisturbed difference frequency of the two laser modes. A small Changing the length of the external resonator leads to this Difference frequency changes. This is the basis for measuring the height profile in Units of the difference frequency. A defined change in length of the external resonator allows calibration, so that a Absolute indication of the height differences is possible.

According to the invention, the second independent measured variable is next to the Difference frequency change the laser intensity change available it allowed locally reflective, refractive and absorptive change on (in) the sample capture.

In the following, the explanations for individual figures Formation of the interferometric near-field arrangement according to the invention be clearly illustrated.

It shows schematically in detail:

FIG. 1 is an interferometric near field according to the invention;

FIG. 2 shows an alternative diagram of the interferometric near-field arrangement according to the invention shown in FIG. 1;

3 shows a section through a standard single-mode fiber ;

FIG. 4a shows a possible technical embodiment;

Figure 4b is a possible technical specifications for applications in which not only high spatial resolution (nm range) additionally high time resolution (femtoseconds) are required.

FIG. 5 shows the result of a measurement of the pump radiation absorption as a function of fiber length, illustrated by the example of a Nd ³⁺ - doped mono mode silica fiber (doping height ~ 1000 ppm Nd ₂ O _3);

Figure 6a shows the change in length of the Verstärkerresonators (height profile of the sample) corresponding change in the difference frequency of the two modes of the fiber laser (parameters:. L _v = 13 cm, m ₂ = 3 x 10 ^5; n _L = 2 × 10 ¹⁴ s ^-1, R ₂ = T ₂ = 0.5, R ₃ = 0.01);

Figure 6b shows the change in length of the Verstärkerresonators (height profile of the sample) corresponding change in the difference frequency of the two modes of the fiber laser (parameters:. L _v = 13 cm, m ₂ = 3 x 10 ^5; n _L = 2 × 10 ¹⁴ s ^-1, R ₂ = T ₂ = 0.5, R ₃ = 0.04);

Figure 7a shows the frequency for understanding the reaction of the Verstärkerresonators to the laser resonant frequency useful scheme for lasers and amplifiers in the event of coincidence position, a laser mode coincides with a mode of the Verstärkerresonators match.

FIG. 7b shows the resonance frequency scheme (like FIG. 7a) in the event of a change in the length of the amplifier resonator; FIG. 7a, which leads to a shift in the laser mode;

Fig. 8 shows the results of a measurement of the small signal gain, the example of Nd ³⁺ -doped single-mode silica fiber of FIG. 5;

Figure 9 shows the percent change of the laser power .DELTA.P / P and the percentage change of effective reflectivity |. .DELTA.R _eff | / R _eff as a function of change in the reflectivity of the mirror .DELTA.R ₃ M ₃ (sample).

The interferometric near-field arrangement shown in FIG. 1 essentially consists of two single-mode quartz glass fibers, one of which is designed as a fiber laser C, the other as a fiber amplifier F. The length of the fiber laser is in a known manner | 5 | chosen so that only two longitudinal modes can oscillate for the selected excitation (pumping) range. The fiber laser resonator is closed off by mirroring M ₁ and an intrafiber reflection grating M ₂ , the fiber amplifier resonator, ie the resonator coupled to the fiber laser by M ₂ , M ₃ . (An elegant, technologically complex implementation of a 2-mode laser is described by SV Chernikov inter alia in Opt. Lett. 18 (23), 1993, page 2023.)

FIG. 2 shows an equivalent scheme of the operation of the arrangement. The fibers are standard single-mode fibers.

The section through such a fiber with the associated refractive index-n profile is shown schematically in FIG. 3. Characteristic fiber data for a laser wavelength ~ 1 µm are: core diameter D ~ 5 µm; Diameter (cladding) N ~ 90 µm, thickness (coating) O ~ 30 µm. The numerical aperture of such fibers is generally ~ 0.2.

Rare earth metals come into question as the doping of the core D. The doping level can be adapted to the requirements of the arrangement to be implemented and can be implemented using known manufacturing methods. In addition to quartz glass, phosphate or fluoride glass can be used as the starting material for fiber production. The choice of the dopants determines the emission wavelength of the laser in a known manner. The doping of the fiber amplifier material must correspond to that of the fiber laser with respect to the dopant; the doping level can be freely selected according to the requirements. Standard semiconductor lasers A, which radiate in basic mode, are to be used as pump lasers. The pump radiation H serves to excite the radiation-amplifying material D. In order to achieve the required pump power density in the fiber core, it is advisable to use the well-known master laser power amplifier (MOPA) combination. The length of the fiber laser C is selected according to the invention such that emission takes place in two longitudinal modes and that a substantial part of the pump radiation (<80%) enters the fiber amplifier F and is converted there into excitation of the radiation-amplifying material. A prerequisite for the functionality of the arrangement is the known fact that the emission wavelength of the MOPA and the absorption band of the dopant must match in the range from 1 to 2 nm. Fiber laser and fiber amplifiers (laser resonator C and external resonator G) are connected to one another in accordance with standard methods via a reflection-free splice E (reflectivity << 10 ^-4 ). The fiber tip K with the light exit opening L is attached to one end of the amplifier fiber using standard methods. It faces Sample I. The distance LI, designated GF, is known to be comparable to the size of the light exit opening L. The radiation losses which are known to occur in fibers which are tapered to a point are taken into account by an attenuation P. The sample, which has a reflectivity, characterized by M ₃ , represents the final mirror of the external resonator. The laser radiation B generated in the fiber laser C and amplified in the fiber amplifier F serves for the contactless control of the distance GF from the sample (range 10 to 100 nm) and for measuring absorptive, refractive and reflective variations on the sample or for writing information into the sample.

A possible technical configuration is shown schematically in FIG. 4a. In the selected example, the fiber of predetermined length is wound around a cylinder Q made of dielectric material and fixed. By applying a voltage to such a cylinder provided with contacts according to known methods, the laser resonator length C can be changed by increasing the cylinder diameter. It is known that the laser power can be stabilized by means of electronic control on the piezo cylinder and the laser modes can be kept at the center of gravity of the amplifier profile. The measurement of the laser intensity can be done with an electronic detection system R _P consisting of receiver S _A , S _E ; Measuring device U, V, R _P : S _A , V; S _E , U take place. Changes in intensity that occur due to absorption in the sample can be detected with a radiation receiver S _A R _{P, A} : S _A , V. By means of a divider mirror T, the laser intensity P on the coupling-in side can be converted into a detection system R _{P, E} : S _E , U can be reflected. This detection system R _PE should consist of a receiver S _E for radiation intensity. Part of the electrical receiver signal should be used in absorption measurements to form the differential intensity DP = P _A - P _E and detected with an oscillograph, cf. G. Herziger et al. f. Appl. Physik 18, 2, 1964, page 67, the other part should be available for electronic frequency analysis U (spectrum analyzer). An advantageous feature of the invention is the tandem pump arrangement for lasers and amplifiers with a powerful diode laser (MOPA).

If the material is to be influenced or measured with a high repetition frequency (<100 MHz) and / or extremely short pulses (femto- / picosecond intensity half-width), the technical configuration of the arrangement according to the invention shown in FIG. 4b should be selected. In this case, the radiation from a short-pulse laser with coupled transport fiber W is coupled into the amplifier fiber, according to known technology, via a directional coupler X, both realized according to known methods. The wavelength of the radiation from the short-pulse laser does not have to be in the spectral amplification range of the fiber amplifier, it should be adapted to the structuring or measurement process.

With the combination of interferometric Near-field arrangement and short-pulse laser can be a defined distance from the material can be set at any point on the material surface, so that too irradiating area always has the same size and structure or measurement of the material can be done with pulses, their temporal Width and repetition frequency can be selected using the coupled short pulse laser are determined.

In FIG. 5, a measurement is to pump radiation absorption as inter alia P. glass fiber and Integr Optics 16 (1), 1997, page 103 |. 6 | was described for an Nd ³⁺ -doped single-mode quartz glass fiber (doping level approx. 1000 ppm Nd ₂ O ₃ ). For a laser in which two longitudinal modes oscillate, a length of £ 10 cm should be provided; compare | 5 |. The fiber length required for two-mode operation is determined according to the in | 6 | described cutting process determined. According to FIG. 5, less than 10% of pump radiation is absorbed for the stated fiber length £ 10 cm. For a given fiber length, the percentage of pump radiation absorbed can be changed by varying the doping level according to known methods. Both the laser frequency and the laser intensity are advantageously available as measurement variables in the near-field arrangement according to the invention. The measuring principle of the laser frequency change according to the invention, which is based on the frequency reaction of the amplifier resonator on the laser resonator, can be described as follows.

If no external resonator is coupled to the laser, the undisturbed natural frequencies of the laser are obtained

m ₁ - integer, n - refractive index

By coupling the external resonator, the laser natural frequency is shifted, so that the perturbed natural frequency applies

ie the total phase shift must again be an integer multiple of π. The phase shift δ at the center mirror M ₂ is

the index L was omitted at R _2.3 .

With ν = ν _L + ν _L and development of the trigonometric functions up to the second order follows:

The standardized representation of the frequency shift of the laser resonance frequency ν _{1, L} as a function of the change in amplifier resonator length is shown in FIG. 6a.

For a 13 cm long amplifier resonator, m ₂ = 2L _v / λ ~ 3 × 10 ⁵ (λ _las = 1.05 × 10 ^-4 cm). The frequency shift Δν _L in this example is ~ ± 6.5 × 10 ⁶ s ^-1 for a change in length of the amplifier resonator length of ± 76 nm. Around the coincidence point = 0, an almost linear frequency shift of the laser resonance frequency ν _{1, L} is obtained Change in amplifier cavity length. The maximum deviation from linearity is ~ 17%.

In Fig. 6b the reflectance of the sample was increased to 4%. For a change in length of the amplifier resonator of ± 48 nm, a frequency shift of the laser mode of ± 1.7 × 10 ⁷ s ^{-1 is obtained} . The maximum deviation from linearity is <5%.

FIGS. 7a, b schematically illustrate how the measurement signal, the size of which is to be determined according to formula (4) or FIGS. 6a, 6b, comes about by forming the difference frequency of the two laser modes.

In Fig. 7a the frequencies of the two laser modes with ν _{1, L} ; ν _{2, L.} The difference frequency that can be measured with an electronic spectrum analyzer (R _p : U) is denoted by ν _{2, L} - ν _{1, L} = Δν _{Diff, L.} For the example shown, Δν _{Diff, L} = 2.1 × 10 ⁹ s ^-1 . The frequency bandwidth of the laser modes is designated δν _L. The index V on the relevant variables was used for the amplifier resonator. In the coincidence position, the laser frequency ν _{1, L} coincides with a mode frequency of the amplifier resonator, ν _{1, V} , ν _{1, L} - ν _{1, V} = 0. The condition is that the frequency of the second laser mode is as far from that of the next one Amplifier resonator mode is removed for their difference

Δν _{2, L-4, V} << δν _{L, V}

applies or numerically for the selected example: 3.2 × 10 ⁸ s ^-1 << 2 × 10 ⁶ s ^-1 . Since ΔL _V = 0 in the coincidence position, it has the value 0 in FIGS. 6a, 6b . If sample I is moved past the tip, ΔL _V changes. The mode frequencies (ν _{1, V} to ν _{4, V} ) of the amplifier resonator thus change in accordance with equation (1), cf. FIG. 7b. (In Eq. (1) replace index L with V). According to Eq. (4) a shift in the laser frequency (ν _{1, L} ) of mode 1, L results. The spectral position ν _{2, L of} the unaffected mode 2, L does not change as long as the condition Δν _{2, L-4, V} << δν _{L, V is} fulfilled. Depending on the current length of the amplifier resonator L _V + ΔL _V , a current difference frequency Δν _{Diff, L} results, which differs from the undisturbed difference frequency Δν ^U _{Diff, L} (coincidence setting, = 0). The change in the differential frequency can be detected electronically (R _p : S _E , U). Using known electronic methods, the measurement signal can either be transformed into a low-frequency range or converted into a DC signal. The resolution limit of electronic spectral analyzers is approx. 100 s ^-1 , which corresponds to a height difference on the sample (change in length of the resonator) of ~ 10 ^-2 nm. To measure such small height differences, a measuring time of 10 to 100 ms is required. The height profile of the sample is shown in the result of the measurement as a location-dependent difference frequency. If a constant distance between fiber tip and sample is required, the measured difference frequency can be used as a manipulated variable for a device for sample displacement (compensation of ΔL _V ). This enables a contactless scanning of sample I. A direct calibration of the frequency change in units of length can advantageously be carried out according to the invention. For this it is only necessary to move the sample by means of piezo-actuators by a defined length .DELTA.L _V from the coincidence position = 0 and out to measure the associated differential frequency. The scanning speed is given by the settling time of the laser, which can be derived from the frequency bandwidth of the modes. It is approximately 0.1 to 1 µs.

The condition Δν _{2, L-4, V} << δν _{L, V} can only be met, especially for the coupled resonator G, if this resonator is attenuated by introducing a radiation-amplifying material D. The frequency bandwidth is given by

(H. Boersch et al. Phys. Lett 4, 1963, page 191).

δν _V is the frequency bandwidth of the undamped resonator, δ ο | V is the bandwidth without amplification, G _o is the amplification coefficient.

. With Figure 8 G _o is cm for L _v = 13: G _o = 1.56 x 10 ^5. With a finesse F of the passive resonator F = 2

(see FIG. 7a), ie δν _V ~ 1 × 10 ⁶ s ^-1 . The bandwidth of the laser is shown in | 5 | indicated with 1-2 × 10 ⁶ s ^-1 , so that the above conditions are very well fulfilled for the selected example.

In addition to the change in difference frequency, the change in intensity of the laser radiation, which can be measured with the power meter R _{p, is} available as a second independent measurement variable. The laser power can be calculated as follows, see AE Siegman, Lasers, Univ. Sci. Books, Mill Valey, Calif. Cape. 12, page 487.

To do this, you must know:
the losses (radiation loss in the fiber tip, fiber losses, the area of the core D, the saturation intensity of the fiber (material constant), the small signal amplification and the effective reflectivity of the coupled resonator.

F - area of the core, T ₂ - transmittance of the coupling-out mirror, I _s - saturation intensity, R ₁ - reflectance of the mirror M ₁ , R _eff - effective reflectance of the mirror combination M ₂ , M ₃ . The effective reflectivity R _eff is a function of the coordination of the two resonators to one another, cf. | 4 |; G ₀ - small signal gain factor.

In Fig. 8 the measured small-signal gain is InG ₀ / L _L = g ₀ shown for in Fig. 5 described example. The determination of G ₀ is in | 6 | explained. For the given example, F = 2.8 × 10 ^-7 cm ² , I _s = 4.5 × 10 ⁴ W / cm ² , G ₀ ~ 7 × 10 ⁷ for L _L + L _V = 18 cm. After | 4 | the influence of the coupled resonator can be taken into account by an effective reflectivity R _eff

FIG. 9 shows the percentage change in laser power ΔP / P and the percentage change in the effective reflectivity | ΔR _eff | / R _eff as a function of the change in reflectance ΔR _{3 of} the mirror M ₃ . In order to demonstrate a change ΔR ₃ ~ 10 ^-2 , a percentage change in power ΔP / P of ~ 5% must be detectable, ie the power measurement with R _p : S _E , V may only be associated with an error <1%. The absolute power for the parameters mentioned is approx. 1 mW. The spatially resolved (100 nm range) absorption changes which can be determined with the measuring arrangement R _p : S _A , S _E , V can be determined according to | 6 | with the characteristic data of the arrangement according to the invention are ≧ 10 ^-3 .

The minimum reflection (ΔR) or refractive index changes (Δn), which can be detected in a spatially resolved manner using the arrangement according to the invention (measuring arrangement R _p : S _E , U), are in the range ΔR ≧ 10 ^-6 ; Δn ≧ 10 ^-5 .

It is conceivable that samples to be examined have areas of different reflectivities in addition to a height profile. Under certain circumstances, this can interfere with the recording of the height profile of the sample or the maintenance of a constant distance from the fiber tip if only the difference in frequency is measured. As the comparison of FIGS. 6a, 6b shows, the measurement of the differential frequency alone is not clear. A change in reflectance can simulate a change in height. An equilateral measurement of the laser power eliminates the ambiguity, since according to FIG. 9 a change in reflectance is directly associated with a change in laser power.

The problem of noise, i. H. the increased spontaneous emission not essential, since only in a very limited Frequency range (frequency bandwidth of the modes) is worked.

For structuring the material through radiation-induced changes it is imperative to keep a constant distance from the material surface to be observed, otherwise the irradiated areas changed differently and / or have different sizes. The regulation on constant Distance takes place via the setting of constant differential frequency and Laser power.

The power of the pump light source A is increased by increasing the diode current with a clock frequency adapted to the test conditions known methods increased (DC operation with superimposed pulses), which is a Increasing the power of the fiber laser can result in the material a radiation-dependent change can be made. Such changes with an adjusted writing speed (0 to 100 MHz) can advantageously with the coupled short pulse laser W run.

The arrangement described for measuring or generating structures in the nm range is to be seen as an implementation variant that shows the possibilities for (laser) interferometric measurement of distances in Nanometer range or the determination and generation of reflective, refractive and absorptive changes in a sample with the highest spatial and time resolution by means of coupled resonators, the one being Fiber laser, the other designed as a fiber amplifier with a tapered end is exploited.

Claims (23)

1. Arrangement for measuring and structuring material (I) with the highest spatial and time resolution in the nanometer range with laser radiation, called interferometric near-field arrangement, which is constructed from a fiber laser (C) and a fiber amplifier (F), using a constant light pump source (A) the radiation-amplifying material (D) of both the laser and the amplifier is excited and the reaction of the fiber amplifier to the fiber laser is exploited, characterized in that the fiber amplifier (F) is spliced onto the fiber laser (C) at one end (E) and at the other Tapered to a fine tip (K) is that the tip (K) faces the material (I) and is at a distance from it, which is determined by the diameter (L) of the tip, and that the material to be examined (I ) is an integral part of the arrangement. 2. Arrangement for measuring and structuring material (I) according to claim 1, characterized in that the fiber laser (C) and the fiber amplifier (F) is designed as a 3-mirror (M ₁ , M ₂ , M ₃ ) tandem arrangement, the Mechanism of action is based on the interferometric coupling of two resonators. 3. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that in the fiber amplifier (F) the radiation of a short pulse laser (W) additionally is coupled. 4. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that  the material (I) as an integral part of the arrangement Frequency repercussions on the fiber laser (F), which acts as a measured variable is used, the measurement signal associated with the frequency feedback is represented as the difference frequency of two laser modes. 5. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the material (I) as an integral part of the arrangement, a Intensity repercussions on the fiber laser (F), which acts as a measurand is used, the measurement signal associated with the intensity feedback presents itself as a change in the intensity of the laser radiation. 6. Arrangement for measuring and structuring material (I) according to Claims 1, 4 and 5, characterized in that the measurands belonging to the frequency and intensity feedback can be used interactively or individually. 7. Arrangement for measuring and structuring material (I) according to Claim 4 characterized in that the difference frequency change can be calibrated in units of length. 8. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the material (I) can be moved past the fiber tip (K) (scanning). 9. Arrangement for measuring and structuring material (I) according to Claims 1, 4 and 7, characterized in that  the height profile of the material (I) as a difference frequency change is ascertainable and can be represented by scanning the material surface. 10. Arrangement for measuring and structuring material (I) according to Claims 1, 4, 7 and 8, characterized in that a difference frequency change by scanning a plane Material surface can be determined, which reflects locally differently. 11. Arrangement for measuring and structuring material (I) according to Claims 1, 4, 5, 7 and 8 characterized in that a difference frequency change and a laser intensity change at Scan a non-flat, locally different reflecting Material surface can be determined. 12. Arrangement for measuring and structuring material (I) according to Claims 1, 4 and 5, characterized in that in the case of a flat, locally differently reflecting material surface a clear characterization of the surface due to the simultaneous Availability of two measurands (differential frequency and Laser intensity change) can take place. 13. Arrangement for measuring and structuring material (I) according to Claims 1, 4 and 7, characterized in that time-dependent changes in the refractive index in the material (I) due to a change the difference frequency can be determined if a characterization according to Height and reflectance was made at this point.   14. Arrangement for measuring and structuring material (I) according to Claims 1, 5 and 8, characterized in that absorption differences when scanning a flat material surface can be determined by changing the laser intensity. 15. Arrangement for measuring and structuring material (I) according to Claims 1, 5 and 14, characterized in that time-dependent absorption differences of the material (I) through a Time dependence of the laser intensity change on previously characterized Points are noticeable. 16. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the fiber laser (C) only emits in two longitudinal modes. 17. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the coordination of the modes of the fiber laser (C) with respect to the center of the Amplifier curve is done by changing the fiber length. 18. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the radiation losses occurring at the fiber tip (K) through the Gain in the fiber amplifier (attenuation) can be compensated. 19. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that  the wavelength of the laser radiation by suitable choice of the dopants radiation-amplifying material (D) and the pump light source (A) can be made changeable. 20. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the pump light source (A) in addition to the DC excitation current with superimposed Current pulses are applied, leading to an increase in fiber laser power to lead. 21. Arrangement for measuring and structuring material (I) according to Claims 1 and 19, characterized in that a radiation-dependent change in the material (I) at constant Distance between fiber tip (K) and material (I) can be made. 22. Arrangement for measuring and structuring material (I) according to Claims 1 and 3, characterized in that the wavelength, pulse width and repetition frequency of the short pulse laser (W) problem-specific can be selected. 23. Arrangement for measuring and structuring material (I) according to Claims 1, 3 and 22, characterized in that highest spatial resolution in the range <100 nm by precise adjustment of the Distance between fiber tip (K) and material (I) combined with the highest Time resolution <10 fs by coupling short-pulse laser radiation into the Fiber amplifier (F) is reached.