WO1992011672A1

WO1992011672A1 - Device for stabilizing a laser light source

Info

Publication number: WO1992011672A1
Application number: PCT/AT1991/000135
Authority: WO
Inventors: Harald Telle
Original assignee: Tabarelli, Werner
Priority date: 1990-12-20
Filing date: 1991-12-18
Publication date: 1992-07-09
Also published as: ATA260890A; AT395497B; EP0563193A1

Abstract

Proposed is a laser light source stabilization device in which the light intensity in the laser resonator changes the resonator's optical path length. The device is particularly suitable for the stabilization of a laser diode (ID). It includes an external resonator (M1, M2, M3), located outside the laser resonator, for the frequency-selective feedback of light from the laser light source (LD) to the laser resonator. Also included is a first control device for the control of at least one parameter affecting the emission frequency of the freely running laser light source (LD) (the injection current in a laser diode). The device further includes a control device for the control of the phase relationship of the light fed back by the external resonator to the laser resonator (feedback phase) relative to the phase of the light in the laser resonator, plus a modulation device for the modulation of a parameter affecting the emission frequency of the free-running laser light source (the injection current in a laser diode). In order to obtain an independent control signal for the control of the feedback phase, the invention calls for a detector (PD1, AMP2, FI1, DBM2, PS2, FD) for the detection, in the light emitted by the laser light source, of a modulation with twice the frequency (2fmod), the detector feeding to the control device (PMG, LIA INT1, PZT2,3) a control signal dependent on the amplitude of this modulation for control of the feedback phase.

Description

Device for stabilizing a laser light source

The invention relates to a device for stabilizing a laser light source, in which the light intensity in the laser resonator changes its optical length, in particular for stabilizing a laser diode, the device comprising:

an external resonator arranged outside the laser resonator for frequency-selective feedback of light from the laser light source into the laser resonator, a control device for regulating at least one operating parameter that changes the emission frequency of the free-running laser light source, in particular the injection current of a laser ¬ diode,

a regulating device for regulating the phase position (feedback phase) of the light fed back from the external resonator into the laser resonator relative to the phase position of the light in the laser resonator,

a modulation device for modulating an operating parameter which changes the emission frequency of the free-running laser light source, in particular the injection current of a laser diode.

For numerous applications, in particular for realizing an interferometer measuring absolute distances, one is on narrow-band and continuously over the largest possible

Interested in the frequency range of tunable laser light sources. In principle, laser diodes are very suitable for this, since they have a large amplification range and can also run in a single longitudinal mode. However, free-running laser diodes, that is to say laser diodes which are only tuned via their operating parameters (injection current or laser temperature) without external optical feedback, cannot be tuned over a large frequency range (in the order of magnitude of 100 GHz and above). On the contrary, such tuning through the laser operating parameters leads to abrupt, non-phase-traceable changes in the emission frequency. In addition, due to the high frequency noise (large line width), the coherence length is more typical, free running Single-mode laser diodes well below one meter, which means that these laser diodes are not suitable, for example, for distance measurements over long distances.

It is already known that the frequency noise of laser diodes can be reduced over a broadband range by weak frequency-selective optical feedback from an external resonator, which results in a substantially narrower line width and thus a higher coherence length. Such a frequency-selective optical feedback from an external resonator, as described, for example, in the work "Frequency stabilization of semiconductor lasers by resonant optical feedback", B. Dahmani et al., Optics Letters, Vol. 12, No .ll, November 1987, pages 876 to 878, a snap-in of the frequency actually emitted by the laser diode onto the resonant center frequency is achieved in a snap-in area around the resonant center frequency of the external resonator. In other words, the frequency-selective optical feedback ensures that even with operating parameters in which the emission frequency of the free-running laser diode (ie without optical feedback from the outside) lies within a certain range (namely the snap-in range) next to the resonance center frequency, an actual emission ¬ frequency, which is practically on the resonance center frequency. In principle, the frequency-selective optical feedback alone is sufficient to stabilize the emission of the laser diode, ie to keep a narrow-band emission line on the resonance * - center frequency of the external resonator. External interference and aging of the laser diode mean that long-term stability can only be achieved by additional electronic controls. Such electronic regulations are described, for example, in the work "Design of an Optically Pumped Cs Laboratory Frequency Standard", E. de Clercq et al., Frequency Standards and Metrology, Springer-Verlag Berlin, Heidelberg 1989, pages 120 to 124. First of all, a regulating device is provided for regulating an operating parameter, in particular the injection current, which changes the emission frequency of the free-running laser diode. This regulation ensures that the emission frequency of the free-running laser diode is at least in the lies around the resonance center frequency of the external resonator, so that the frequency-selective optical feedback is able to guide the actual emission frequency to the resonance center frequency of the external resonator. If the emission frequency of the free-running laser diode corresponding to the current operating parameters does not exactly match the resonance center frequency of the external resonator, the actual emission frequency (although only slightly due to the frequency-selective optical feedback) deviates somewhat from the resonance center frequency. This deviation can be detected and a control signal can be obtained from it in order to regulate the operating parameters of the laser diode in such a way that the emission frequency of the free-running laser diode always corresponds exactly to the resonance center frequency of the external resonator.

In addition to the control of the operating parameters of the laser diode described above, a control device for controlling the phase position of the light fed back from the external resonator into the laser resonator relative to the phase position of the light in the laser resonator is also necessary. This regulation of the feedback phase can take place in a manner known per se by regulating the distance from the laser diode to the external resonator, for example by guiding the light beam via a piezoelectrically adjustable mirror. At this distance, the absolute size is less important than the proportion deviating from an integer multiple of half the wavelength (feedback phase). Two independent control loops are required for the control of the operating parameters and the control of the feedback phase described above. The aforementioned work by E. de Clercq et al. suggests modulating the injection current of the laser diode. This modulation induces frequency modulation in the light emitted by the laser diode. This frequency modulation is detected in the transmission of the external resonator and a control signal is determined therefrom for regulating the feedback phase. In order to achieve an independent control signal for regulating the operating parameters of the laser diode (regulating the emission frequency of the free-running laser diode to the latching area or precisely to the resonance center frequency of the external resonator), according to the proposal by de Clercq, the light fed back into the laser resonator is amplitude-modulated with a second frequency via an acousto-optical modulator. By demodulating the light transmitted by the external resonator at the total frequency (injection current modulation frequency plus modulation frequency of the acousto-optical modulator), a control signal is obtained for regulating the injection current and thus the emission frequency of the free-running laser diode. Experiments by the applicant have shown that even with expensive acousto-optical modulators, only very small signal-to-noise ratios can be achieved for the control signal. It is only possible to achieve small control bandwidths, which are at most sufficient to stabilize a fixed frequency laser, as also described in the article by de Clercq et al. the case is.

However, one is particularly interested in light sources that can be tuned continuously (phase-traceable) over a large frequency range. For this, the resonance center frequency of the external resonator can be tuned. So that the optical feedback is able to keep the actual emission frequency of the laser diode always at this resonance center frequency, the emission frequency of the free-running laser diode (for example via the injection current) must also be controlled via the first regulation described above, so that the emission frequency of the free-running laser diode is always in the snap-in area around the resonance center frequency. It is also necessary to regulate the feedback phase in an independent, second control loop (cf. Dahmani's work mentioned at the beginning). While the formation of a control signal from the deviation of the actual emission frequency of the laser diode from the resonance center frequency of the external resonator and thus the regulation of the frequency of the free-running laser diode (for example via the injection current) is not a problem, the provision of a second, independent control signal for the feedback phase for realizing a light source system that can be tuned quickly, broadband and, above all, phase trackable (ie without mode jumps). It is therefore an object of the invention to provide a device for stabilizing a laser light source of the type mentioned at the outset, with which a long-term stable or fast, broadband and phase-traceable light emission of small line width can be achieved.

This is achieved according to the invention in that a detector device for detecting a modulation with twice the modulation frequency is provided in the light emitted by the laser light source, this detector device emitting a control signal dependent on the modulation stroke of the aforementioned modulation to the control device for regulating the feedback phase .

The measure according to the invention makes it possible, in addition to the regulation of the emission frequency of the free-running laser light source, to obtain an independent control signal for regulating the feedback phase by changing an operating parameter, and thus to implement a laser light source which is stable over the long term or can be quickly and broadly phase-traced.

The basic idea is the following: By modulating an operating parameter of the laser light source, which influences the emission frequency of the free-running laser light source, frequency modulation is generated in the light emitted by the laser light source. When using a laser diode, for example, the injection current can be superimposed on a small, high-frequency alternating current. The stroke of this frequency modulation is strongly suppressed in comparison to a free-running laser diode by the frequency-selective optical feedback from the external resonator, for example by a factor of 50. The alternating current amplitude superimposed on the injection current can be chosen, for example, so that the modulation index, i.e. the ratio of the modulation stroke to the modulation frequency is small compared to 1 when the feedback parameters are optimally set. In this case, the optical frequency spectrum of the laser light source consists of a central carrier and side bands arranged symmetrically thereto at a distance from the modulation frequency and integer numbers Multiples of it. The relative strength of the sidebands is determined by the square of the order corresponding to the Bessel function, the modulation index being the argument of the Bessel function.

With perfect frequency modulation, there is no alternating component with the modulation frequency or multiples thereof in the photocurrent of a photodiode (photodetector) that detects the light emitted by the laser light source. This is due to the fact that the beat components resulting from the superimposition of the individual side bands with one another or with the carrier add up to zero. In the first-order sidebands, the beat signals between the carrier and the right sideband and the carrier and the left sideband are the same in magnitude, but have an opposite sign. The compensation of the beat signals with twice the modulation frequency can be imagined as follows: The beat signal between the two first order sidebands is compensated for by the constructive superposition of the two beat signals of the right and left second order sideband with the carrier. However, this compensation no longer exists if the strength of one of the two second-order sidebands deviates from the value prescribed by the Bessel function of the second order for the modulation index in question.

If, when regulating the frequency actually emitted by the laser diode (system frequency) to the resonance center frequency of the external resonator, the feedback phase drifts away from its optimal value (in which the field vector of the feedback light is perpendicular to the field vector of the light in the laser resonator) , an amplitude modulation with twice the modulation frequency appears in the light (reflected by the external resonator). A possible explanation for this is that if the feedback phase is not optimal, at least one second-order sideband comes outside the snap-in area (as already mentioned, the snap-in area is due to the maximum deviation of the laser diode frequency from the resonator center frequency of the external resonator, for which, with a suitable feedback phase, there is still an effective noise reduction or narrowing of the line width). If a second-order sideband gets outside the snap-in area, the relevant second-order sideband is no longer suppressed as efficiently by the optical feedback from the external resonator; the compensation of the sideband contributions existing with ideal frequency modulation is no longer possible, and it In the light emitted by the laser diode, an amplitude modulation is created with twice the modulation frequency, which is detectable in the photocurrent. According to the invention, a detector device can now be provided for the detection of this amplitude modulation at twice the modulation frequency, a control signal for the control device for controlling the feedback phase being obtained from the size of this amplitude modulation (modulation stroke). The control device then automatically adjusts the feedback phase until the amplitude modulation at twice the modulation frequency assumes a predetermined value or a minimum at which the feedback phase is optimal.

Another effect could also contribute to the amplitude modulation with twice the modulation frequency, which occurs from the external resonator: The light emitted by the laser diode is frequency-modulated due to the modulation of the injection current, the intensity of the first and second order sidebands also depends on the feedback phase. With an optimal feedback phase, all side bands are strongly suppressed. If the feedback phase deviates from the optimal value, the intensity of the sidebands will increase, and with it the modulation deviation of the frequency modulation. Even if the sidebands of the first and second order were influenced so "evenly" by a changing feedback phase of the optical feedback that with pure frequency modulation the light emitted by the laser diode would remain, one could use the light reflected by the external resonator see an amplitude modulation dependent on the feedback phase at twice the modulation frequency. This is due to the fact that the external resonator carrier of the frequency-modulated light is reflected less strongly than the sidebands. While this has no effect in the reflected light at the simple modulation frequency, the attenuation of the carrier results in an amplitude modulation with the second modulation frequency in the reflected light. The modulation stroke of this amplitude modulation reflects the modulation stroke of the frequency modulation impinging on the resonator from the laser diode, which in turn depends on the feedback phase.

According to a preferred embodiment of the invention, it can be provided that the detector device comprises a photodetector and a mixer connected downstream of it, which, in addition to a signal input for the possibly amplified photocurrent from the photodetector, has a reference input for receiving a signal coming from the modulation device and via a frequency - Has doubler-guided reference signal with twice the modulation frequency. It is thus possible to convert the alternating current signal emitted by the photodetector into the baseband and to use it as an error signal for the feedback phase control loop. In principle, regulation via edge stabilization is possible. However, it is more favorable to use a modulation method to determine a control signal for the feedback phase control. For this purpose, according to a preferred embodiment of the invention, it is provided that a phase modulation device is provided for small-stroke modulation of the feedback phase, and that a phase-sensitive detector device is also provided which consists of a momentary modulation stroke of the (amplitude) modulation, which oscillates with the phase modulation frequency in the case of the signal representing the double modulation frequency, provides a control signal of odd symmetry for the control device for controlling the feedback phase. While the current modulation frequency is advantageously in the order of 10 to 100 MHz, the phase modulation frequency of the feedback phase caused by the phase modulation device is significantly lower and is preferably in the kHz range. The feedback phase can be modulated in a simple manner by the laser beam being see laser diode and external resonator is guided over a piezoelectrically adjustable mirror, which is periodically adjusted with the phase modulation frequency by small fractions of a wavelength. By phase-sensitive detection (for example with a lock-in amplifier) of this low-frequency alternating component in the baseband signal, which represents the instantaneous modulation stroke of the (amplitude) modulation at twice the modulation frequency, a control signal (error signal) with odd symmetry is obtained, which the control device for controlling the feedback phase is supplied. This

Control signal occurs as a product of two modulations carried out from the outside and is therefore largely immune to technical faults, offsets, etc. The control device for the feedback phase then follows the control signal, for example the piezoelectrically adjustable mirror, as a function of this control signal that the feedback phase is optimal.

The control of the feedback phase just described, with evaluation of an (amplitude) modulation occurring at twice the modulation frequency, has a wide control bandwidth and is therefore particularly suitable for systems in which the emission frequency is in the order of 100 GHz over a tuning range and should be phased through (and as quickly as possible). Such tuning can be carried out by tuning the resonance center frequency of the external resonator, a first control device ensuring that the operating parameters of the laser light source are carried along in such a way that the emission frequency of the free-running laser diode is always within the latching range now moving with the resonance center frequency lies. A second control loop keeps the feedback phase at its optimum value while tuning the frequency.

The regulation of the feedback phase according to the invention by evaluating those appearing at twice the modulation frequency

(Amplitude) modulation sets a modulation of the operating parameters determining the emission frequency of the free-running laser diode or one of these operating parameters (for example the Injection current) ahead. As already mentioned, this modulation results in frequency modulation of the light emitted by the laser diode, the frequency spectrum naturally also having first-order sidebands lying symmetrically thereto in addition to the carrier frequency. In the case of an ideal frequency modulation, the beat signals between the carrier and the left first-order sideband and between the carrier and the right first-order sideband cancel each other out exactly. Due to this "balance", no amplitude modulation is initially visible. However, if the carrier frequency deviates from the resonance center frequency of the external resonator, then the light reflected from the resonator interferes with the said "balance" between the carrier and first-order sidebands, and this results in an amplitude modulation that can be detected for a photodetector the simple modulation frequency. According to a preferred embodiment of the invention, phase-sensitive rectification can be used to emit a control signal which is dependent on the modulation stroke and the phase position of the aforementioned amplitude modulation with the simple modulation frequency, for regulating at least one operating parameter of the laser light source.

There are several options for controlling one or more operating parameters of the laser diode as a function of a deviation of the actually emitted frequency from the resonance center frequency of the resonator, for example a polarization-optical phase bridge method or an intensity difference method. The above-mentioned modulation method, in which one evaluates an amplitude modulation with the simple modulation frequency in the light reflected by the resonator, is particularly favorable in the present case, however, because according to the invention the frequency of the free-running laser diode is modulated anyway via the operating parameters by the inventive one To be able to carry out feedback phase control. The first-order sidebands also automatically occur, which ultimately lead to an amplitude modulation together with the carrier in the light reflected by the resonator if the actual emission frequency of the laser diode deviates from the resonance center frequency of the external resonator, with the modulation stroke the. mentioned Amplitude modulation with the simple modulation frequency receives a dispersion curve-like course over the frequency.

With a test setup of the invention that has not yet been optimized, a continuously phase-traceable frequency tuning range of over 100 GHz has already been achieved. By optimizing the components used and in particular by using a so-called mode selector, the tuning range can be expanded many times over. Such a mode selector is an external reflector which is arranged in the vicinity of the laser diode resonator. If the frequency is to be tuned, the distance of this reflector from the laser diode resonator must also be traveled. This requires a third control loop (the first control loop controls the injection current, for example, so that the frequency of the free-running laser diode lies in the snap-in range around the resonance center frequency of the external resonator; the second control controls the feedback phase). For the third regulation of the external reflector (mode selector) now provided according to a preferred embodiment, the latter can be mounted on a piezo ceramic and the distance to the laser diode can be modulated, the modulation frequency being of the order of magnitude of approximately 100 Hz . It was observed that the power of the light transmitted by the partially transparent mode selector depends on the position of the mode selector and that the

Modulation receives a transmission curve similar to a resonance line. The transmitted light is thus modulated, and a control signal for tracking the mode selector can be obtained via phase-sensitive rectification.

The modulation of the mode selector induces a low frequency modulation in the laser light source. To avoid this, according to a preferred embodiment of the invention, provision can be made for a device for modulating at least one operating parameter of the laser light source with the reflector modulation frequency to be provided in the laser by the reflector modulated at a distance from the laser light source to compensate for light source-induced frequency modulation. Further details and advantages of the invention are explained in more detail in the following description of the figures.

1 shows a schematic representation of an exemplary embodiment of the device according to the invention. 2 shows a diagram to show the frequency response of a laser diode with and without frequency-selective optical feedback and to show the snap-in area. 3 shows the essential components of a frequency spectrum of a frequency-modulated oscillation. 4 shows a piezoelectrically adjustable external mode selector and its control loop.

In the embodiment shown in Fig. 1, the actual optical laser system is arranged in the area 1 surrounded by dashed lines and essentially comprises a laser diode LD and a V-shaped resonator with mirrors Ml, M2 and M3, with the light path between the laser diode LD and external resonator, a mirror 3 and a collision lens L is arranged. The laser diode can be, for example

GaAlAs laser diode with an emission wavelength of approx. 830 nm and an output power of approx. 15 mW. The external resonator M1, M2, M3 is a so-called Fox-Smith resonator, in which only the internal field passing through the folding mirror M2 is fed back into the laser light source LD, while the light reflected at the folding mirror is fed into the actual output light beam of the optical laser system goes. Alternatively, other resonators could also be used, in which the light reflected at the coupling mirror does not get directly back into the laser diode. For example, a tilted convocal Fabry-Perot etalon would be suitable. In an experimental setup, the resonator length was approx. 30 cm, the free spectral range approx. 500 MHz and the finesse approx. 250. The feedback level was approx. -45 dB. The resonator mirror Ml is mounted on a miniature loudspeaker LS, with which a change in length and thus a change in the resonance center frequency of the external resonator M1, M2, M3 is possible without substantially misaligning the resonator. In principle, other adjustment mechanisms for a resonator mirror, for example piezoelectric adjustment elements.

The following essential frequencies are defined for the present application: f _F denotes the frequency of the free-running laser diode, that is to say the frequency with which the laser diode would run without optical feedback from an external resonator given given operating parameters (injection current, temperature) . f _g denotes the system frequency, ie the frequency actually emitted from the external resonator and the set operating parameters when there is feedback. (Since the frequency actually emitted is slightly frequency-modulated despite optical feedback from the external resonator, f _s more precisely denotes the central carrier frequency of the light actually emitted by the laser diode.) F _R denotes that due to the length of the resonator (and, if appropriate by the refractive index of the medium contained therein) defined resonance center frequency of the external resonator.

The frequency-selective optical feedback from the external resonator M1, M2, M3 is in principle able, without additional electronic control loops, to keep the system frequency f _g at the resonator center frequency f _R , provided the operating parameters (injection current and temperature of the laser diode) are such are set that the frequency of the free-running laser diode defined thereby lies in the snap-in area around the resonance center frequency f _R of the external resonator, ie does not deviate too far from the resonance center frequency of the external resonator. This is explained in more detail below with reference to FIG. 2.

2, the system frequency f _g (ie the actually emitted frequency) is plotted against the frequency of the free-running laser diode. The origin of the axis cross shown in FIG. 2 lies at the resonance center frequency f _{R of} the external resonator. Without frequency-selective optical feedback, the system frequency changes linearly with the frequency of the freewheeling the laser diode, as indicated by the dashed line 4. The straight line 4 in FIG. 2 does not run exactly below 45 * to the two axes only because a different division is used on the X-axis and the Y-axis. A unit of length on the X axis corresponds to five times the frequency of the same unit of length on the Y axis.

The frequency curve indicated by line 5 is obtained by frequency-selective optical feedback from an external resonator. Are all components of the system optimal

"Locked", the system frequency f _s corresponds to the frequency of the free-running laser diode f _F and the resonance center frequency f _{R of} the external resonator. 2. The frequency of the free-running laser diode now deviates from the resonance center frequency of the external resonator, for example due to a slight change in the injection current or the temperature, the actual selective changes due to the frequency-selective optical feedback ¬ Lich emitted system frequency within the 1 snap range hardly. The frequency-selective optical feedback thus ensures that the actually emitted system frequency fg is kept essentially at the resonance center frequency of the external resonator even with small deviations in the frequency of the free-running laser diode. The effect of frequency-selective optical feedback can be explained in a simplified manner as follows: If all frequencies f _g , f _F and f _R match, the field vector of the feedback light is less than 90 "to the field vector in the laser resonator and this results The feedback phase is therefore 90 °. If the system frequency now deviates from the resonance center frequency, there is a change in the feedback phase, with the field vector of the feedback light now being in or against The phase of the field vector of the laser resonator light stands, resulting in a change in intensity in the laser resonator, which changes the refractive index of the laser resonator and thus its optical length, the emission frequency changing so that it changes again Direction of the resonance center frequency f _R moves and again the phase position of 90 "between the laser diode resonator field and the feedback field.

In practice, however, this purely optical frequency-selective feedback is not sufficient to keep the laser diode at the resonance center frequency for a long time. In particular, if one tunes the resonance center frequency and wants the frequency-selective optical feedback to "pull along" the system frequency with this tuning, additional electronic control loops are necessary to ensure, on the one hand, that the frequency f _F determined by the operating parameters is free ¬ fenden laser diode is at least in the latching area 1 (preferably controlled to the resonance center frequency f _R ). On the other hand, an additional control of the feedback phase is also necessary, which supports the phase control which automatically proceeds as a result of the optical feedback from the external resonator and ensures that the feedback phase is just such that the feedback field is at 90 ° to the field in the resonator of the laser diode.

A first control device regulates the injection current of the laser diode LD via the current control SR in such a way that the system frequency f _{s is kept} at the resonator center frequency f _{R of} the external resonator M1, M2, M3. (The temperature of the laser diode only needs to be roughly pre-controlled.) This control device comprises a modulation device for modulating the injection current of the laser diode. The modulation device essentially consists of the HF synthesizer SY, which supplies a small AC component with a modulation frequency of 35 MHz. The modulation frequency is _denoted by f _{mθ (j} . This injection _current modulation generates a frequency modulation of the light emitted by the laser diode, so that the emission spectrum essentially looks as it is shown in FIG. 3: in addition to the system carrier frequency f _s there are symmetrical first and second order sidebands at a distance of the single and double modulation frequency f _{mθ (j} . higher order sidebands can be neglected and are therefore not shown in detail. A pure frequency modulation is - as is already mentioned at the beginning - not detectable by a photodetector. The external resonator M1, M2, M3, however, acts as a frequency discriminator, wherein in the light reflected by the coupling mirror M2, an amplitude modulation oscillating with the simple modulation frequency f _{m0 (j)} occurs when the system carrier frequency fg lies next to the resonator center frequency f _R This amplitude modulation can be detected by the photodetector (for example an Si-PIN photodiode PD1), the resulting photocurrent representing the amplitude modulations of the detected light. At this point it should be mentioned that the actual useful light is, for example, not shown Beam splitter can be branched off in front of the photodetector PD1.

The photocurrent from the photodetector PD1 is amplified in the amplifier AMP1 and synchronously demodulated in a double-balanced mixer DBM1. The reference signal required for this with the modulation frequency f _{mθ (j} is also taken from the RF synthesizer SY. An RF phase shifter PSl serves to establish the required quadrature relation between the two mixer inputs. PSl can also be selected The control signal (error signal) present at the mixer output, which is proportional to the deviation of the system frequency f _g from the resonator center frequency f _R , is passed through an

Current control SR led to the laser diode. The first control loop is now closed.

In addition to the regulation of the system frequency to the resonator center frequency just described, according to the invention, the feedback phase of the light fed back from the external resonator into the laser resonator is regulated relative to the phase position of the light in the laser resonator, with the second control signal required for this being obtained in the light emitted by the laser diode after a Amplitude modulation at twice the modulation frequency (2f _mod ) is observed. As already explained above, the component oscillating at twice the modulation frequency contains information about the deviation of the feedback phase from its optimal value. The detector device for the detection of this amplitude modulation with twice the modulation frequency comprises as essential elements the photodetector PD1 and the mixer DBM2, which, via the frequency doubler FD from the HF synthesizer SY, oscillates at twice the modulation frequency (2f _mod = 70 MHz) Reference signal is supplied. A phase shifter PS2 is provided to set the necessary quadrature phase relation between the two inputs of the mixer DBM2. The output signal of the mixer DBM2 reflects the modulation stroke of the amplitude modulation with twice the modulation frequency 2f _{mo £ j} and is used according to the invention to regulate the feedback phase. Before this control device of the feedback phase is described, it should also be mentioned that an electronic filter FI1 is provided in front of the mixer DBM2, which essentially only amplifies the component of the amplifier AMP2 which oscillates at twice the modulation frequency 2f _{mo (j} If a very selective filter is used, instead of the mixer DBM2, a squaring device could also be provided (not shown), in which the signal modulated at twice the modulation frequency is multiplied by itself in order to adjust the modulation stroke of this Am ¬ to provide corresponding output modulation component.

To regulate the feedback phase, the piezo actuator PZT2, on which the mirror 3 is attached, is modulated with a small stroke with a frequency of approximately 1 kHz coming from the phase modulation generator, so that the feedback phase is also modulated with a small stroke with this phase modulation frequency. The phase modulation frequency is substantially _lower than the current modulation frequency f _mocjf, so that there is no disruptive mutual influence. The modulation stroke is, for example, in the 10 nm range, which corresponds to a feedback phase stroke of approximately 100 mrad. The 1 kHz component in the DBM2 output signal, which reproduces the momentary modulation stroke of the amplitude modulation at twice the modulation frequency 2f _mod , is now synchronously demodulated with the aid of a phase-sensitive detector device in order to provide a control signal with odd symmetry To get control of the feedback phase. In the present exemplary embodiment, the phase-sensitive detector device is a lock-in amplifier which receives the signal coming from the mixer DBM2 at a signal input and which furthermore has a reference input via which a reference signal oscillating with the phase modulation frequency is output the phase modulation generator is received. After passing through a summer S and an integrator INT2, the signal generated via the lock-in amplifier is passed to the input of the high-voltage amplifier (not shown) of the piezo actuator PZT2 in order to adjust the mirror 3 and thus the feedback phase to the optimum value. This closes the feedback phase locked loop.

As can be seen from FIG. 1, the detector device for detecting the component of the modulated light which oscillates at twice the modulation frequency is arranged in the beam path of the light reflected by the external resonator M1, M2, M3. It is thus possible to use one and the same photodetector PD1 for regulating the system frequency via the operating parameters and regulating the feedback phase.

By means of the two control loops described, the system remains locked onto the resonator center frequency of the external resonator practically indefinitely, even if the resonance center frequency of the resonator is fast (in the order of a few GHz per second) and wide (in the order of 100 GHz and above) is tuned. Since the control stroke of the first control circuit is limited (to the injection flow), the temperature of the laser diode must be roughly controlled if the resonator center frequency is tuned further. The first control circuit then ensures through precise control of the injection current that the frequency of the free-running laser diode remains exactly at the resonator center frequency. This enables a narrow-band and phase-traceable (ie without mode jumps) tunable laser light source to be implemented, such as that used in interferometers for absolute distance measurement. With such an absolute interferometer the construction of a two-beam interferometer, one arm representing the measuring section. The number of interference strips passing through at the location of the receiver as a result of tuning the emission frequency is counted. Simultaneously, one measures the amount by which the air wavelength of the emitted radiation in the surrounding medium changes during the tuning process. The absolute amount of the measurement distance then results from half the number of interference fringes divided by the difference between the air wave numbers (1 / air wave length) before and after tuning the emission frequency.

In the device shown in FIG. 1, the resonance center frequency of the external resonator M1, M2, M3 is adjusted via a movable mirror M1, which is mounted on a loudspeaker LS. When tuning the resonator center frequency of the external resonator M1, M2, M3, the operating parameters of the laser diode must be regulated or controlled so that the emission frequency of the free-running laser diode always remains in the pulling range of the now changing resonance center frequency. The laser diode temperature, which is determined, for example, by a Peltier element 6, is particularly suitable for coarse control for tuning over a large frequency range. In the exemplary embodiment shown, a periodic or linear temperature setpoint curve is specified via a generator G, for example. The temperature controller then carries out a synchronous adjustment of the temperature of the laser diode and the position of the resonator mirror Ml. Such a synchronous adjustment (with control) is of course not perfectly possible. The remaining small deviations between the system frequency fg and the resonance center frequency f _{R of} the external resonator can, however, be quickly and exactly corrected by regulating the injection current as described above.

It should also be mentioned that by choosing a special geometry, in which the distance between the laser diode and the coupling mirror M2 corresponds exactly to the distance between the mirrors M2 and M3, it can be achieved that a feedback phase, once set, can also be achieved when moving Ml remains. Works in practice this feed-forward compensation is of course not perfect. By contrast, the remaining errors, which can arise, for example, from dispersion effects or from small temperature-dependent changes in the laser diode emission axis, are so small that they can easily be corrected by the feedback phase control. An experiment has shown, for example, that when the system frequency is tuned over a range of 200 GHz, the feedback path length between the external resonator and the laser diode only had to be readjusted by less than half a wavelength, which is easy with a simple piezoelectric actuating element PZT2 is possible.

In order to further increase the phase-traceable tuning range of the device, it can be provided according to a preferred embodiment of the invention that, in addition to the external resonator M1, M2, M3, an external reflector MS acting as a mode electrode is arranged outside the resonator of the laser light source LD which reflects back at least part of the light into the laser resonator.

In particular if one wants to tune the frequency of the overall system, the distance of this mode selector from the laser resonator must also be controlled. For this purpose, a third control loop is provided, which is shown schematically in FIG. 4. The position of the mode selector MS is in this case determined by the amplifier OP2 via the Operations¬ voltage supplied through a voltage applied to the input of this operational amplifier OP2 bias ^and bias ^{ann e} i ^ne mode m to be selected manually. To obtain a control signal with which the distance of the mode selector from the laser diode can be regulated, a reflector modulation frequency of the order of magnitude of approximately 100 Hz is superimposed on this bias voltage. The distance of the reflector (mode selector MS) is thus modulated. The reflector MS is designed to be partially transparent and the transmitted power depends on the distance of the reflector from the laser diode. The light transmitted by the reflector is thus also amplitude-modulated with the reflector modulation frequency in the order of 100 Hz. The photodetector PD2 detects and this amplitude modulation leads to a corresponding of the current signal to the signal input S1 of a lock-in amplifier. The reflector modulation frequency U _mod is fed to the reference input RI. The lock-in amplifier now demodulates the amplitude modulation component at the reflector modulation frequency (phase-sensitive rectification) and emits a control signal which, after integration in the correspondingly connected operational amplifier 0P2, is fed to the input of the operational amplifier OP2, which is then fed via the piezo Element PZT sets the mode selector to the correct distance from the laser light source. The mode selector control loop is thus closed.

Since the modulation of the mode selector reflector MS induces frequency modulation in the laser diode, which may be disruptive, according to a preferred embodiment of the invention, a device can be provided which modulates the current of the laser diode slightly with the reflector modulation frequency and thus the through to compensate for the modulated reflector MS-induced frequency modulation. This device CC for current modulation supplies an output current which is superimposed on the injection current of the controls shown in FIG. 1.

A neutral filter NF prevents disruptive reflection from the photodetector into the laser diode LD.

5 shows a typical course of the electrical signal on the photodiode PD1 at the double modulation frequency (S ₂ f _mθ i) as a function of the feedback _phase RKP, ie as a function of the position of the piezo element PZT2 on which the Mirror 3 is mounted. With the regulation described above, regulation to the extremum A takes place by means of a modulation method (modulation of the feedback phase with 1 kHz by PMG). The advantage of this modulation method is that fluctuations in the signal level do not disturb. The control device for the feedback phase can therefore always set the position of the piezo element PZT2 exactly such that the signal assumes the extremum A at twice the modulation frequency S ₂ f- _{mo (} _) and the feedback phase is thus correct. A simplified embodiment of the device according to the invention is shown in FIG. 6. This is simplified in two ways compared to the previously described device. First of all, it should be mentioned that in the device shown in FIG. 6, the first control circuit which regulates the operating parameters of the laser diode LP (ÄMP1, DBM1, PSI, SY, INTI, SR) is the same as that in FIG 1 shown embodiment and here in Fig. 6 is generally designated by the reference number 10.

A simplification in the embodiment shown in FIG. 6 is firstly that by using a highly selective filter FI1 at twice the modulation frequency S _fmod (70 MHz) the mixer DBM2 of FIG. 1 is omitted. Second, there is no modulation of the feedback phase. Rather, a "static control" takes place on the feedback phase B shown in FIG. 5, in which the signal is regulated at the double modulation frequency ^s 2fmod ^{to the Pe} ζT ^el . For this purpose, a rectifier 11 is connected downstream of the filter FI1, the output of which leads to a differential amplifier 12. An offset signal corresponding to the level C is fed to the second input of the differential amplifier 12. The output of the differential amplifier 12 then controls the piezo element PZT2 and thus defines the position of the feedback phase.

In the exemplary embodiment shown in FIG. 7, there is likewise no modulation of the feedback phase, but rather regulation to the value B according to FIG. 5. Instead of or in addition to the filter FI1 at twice the modulation frequency S ₂ f _{mocj, there} is a double-balanced mixer DBM2 is provided, which on the one hand receives the signal coming from the photodiode PD1 and on the other hand receives a reference signal with twice the modulation frequency S ₂ f _moj . The output of this mixer DBM2 controls the piezo element PZT2 via the integrator INT2 and thus regulates the feedback phase. Compared to the exemplary embodiment shown in FIG. 6, in the exemplary embodiment shown in FIG. 7, reduced demands are made on the filter FI1 at twice the modulation frequency. If necessary, it can even be omitted entirely. In the embodiments according to FIGS. 6 and 7, fluctuations in the signal level interfere with the control. The modulation method already used in the exemplary embodiment in FIG. 1 (modulation of the feedback phase via PMG), which is used to regulate not to a certain level of the photoelectric signal present at the second modulation frequency, but to an extremum A, is in this regard more advantageous and safer. In the embodiment shown in FIG. 8, a modulation method is therefore also used, as in the embodiment according to FIG. 1. Compared to FIG. 1, however, there is a simplification in that no mixer DBM2 is provided. Accordingly, the frequency doubler FD and the phase shifter PS2 can also be omitted. Only a highly selective filter FI1 is provided at twice the modulation frequency, which is followed by a rectifier 11 '.

The rest of the structure corresponds essentially to that of FIG. 1. In the exemplary embodiment shown in FIG. 8, the feedback phase is also regulated by means of a modulation method by regulating the photoelectric signal present at the second modulation frequency to the extremum A of FIG. 5.

The invention is of course not limited to the exemplary embodiments shown. Laser diodes are certainly advantageous laser light sources. What is essential, however, is the property of the light source that its optical length changes when the light intensity changes in the laser resonator. This is the prerequisite for the frequency-selective optical feedback to allow the emission frequency to snap to the resonant center frequency of the external resonator. As already mentioned, other resonators, for example tilted confocal Fabry-Perot etalons, can also be used instead of the V-shaped resonator shown. The adjustment of the feedback phase could also be done, for example, via a regulated phase shifter element in the beam path between the external resonator and the laser light source.

Claims

Patent claims:

1. Device for stabilizing a laser light source (LD), in which the light intensity in the laser resonator changes its optical length, in particular for stabilizing a laser diode, the device comprising:

an external resonator (M1, M2, M3) arranged outside the laser resonator for frequency-selective feedback of light originating from the laser light source (LD) into the laser resonator,

a control device (PD1, AMP1, DBMl, PSl, INTl, SR) for controlling at least one operating parameter that changes the emission frequency of the free-running laser light source (LD), in particular the injection current of a laser diode (LD),

- A control device (PMG; LIA; 2; INT2, PZT2; 3) for regulating the phase position (feedback phase) of the light fed back from the external resonator (M1, M2, M3) into the laser resonator relative to the phase position of the light in the laser resonance - goal,

a modulation device (SY) for modulating an operating parameter which changes the emission frequency of the free-running laser light source (LD), in particular the injection current of a laser diode (LD) with a modulation frequency (f _mod ), characterized in that a detector device (PD1, AMP2 , FIl, DBM2, PS2, FD) for detecting a modulation with twice the modulation frequency (2f _mod ) in the light emitted by the laser light source, this detector device sending a control signal dependent on the modulation stroke of the above-mentioned modulation to the control device ( PMG; LIA; S; INT2; PZT2; 3) for regulating the feedback phase.

2. Device according to claim 1, characterized in that ei¬ nem photodetector (PD1) of the detector device optionally with the interposition of an amplifier (AMP2) an electronic frequency filter (FIl) for filtering out the the double modulation frequency (2f _mod ) oscillating component of the light originating from the laser light source (LD) is connected downstream.

3. Device according to claim 2, characterized in that de frequency filter is followed by a squaring device in which the modulated signal with twice the modulation frequency (2f _mod ) is multiplied by itself.

4. Device according to claim 1 or 2, characterized in that the detector device comprises a photodetector (PD1) and a mixer connected downstream (DBM2) which, in addition to a signal input for the optionally amplified photo current from the photodetector (PD1), a reference input Reception of a reference signal originating from the modulation device (SY) and conducted via a frequency doubler (FD) at twice the modulation frequency (2 ^^).

5. Device according to one of claims 1 to 4, characterized ge indicates that the modulation frequency (f _m o _d ) i ^{n is of the} order of 10-100 MHz.

6. Device according to one of claims 1 to 5, characterized in that a phase modulation device (PMG) is provided for small-stroke modulation of the feedback phase, and that a phase-sensitive detector device (LIA) is provided, which consists of a with the phase mod ¬ oscillation frequency, the momentary modulation stroke of the modulation at twice the modulation frequency

(2f _{] mod} ) reproducing signal provides a control signal of odd symmetry for controlling the feedback phase.

7. Device according to claim 6, characterized in that the phase-sensitive detector device, preferably a lock-in amplifier (LIA), has a signal input for receiving a modulation stroke of the modulation at twice the modulation frequency (2f _mod ) reproducing signal and furthermore has a reference input for receiving a reference signal oscillating with the phase modulation frequency.

8. Device according to claim 6 or 7, characterized in that the phase modulation frequency is substantially lower than the modulation frequency (f _mo ) of one or more operating parameters of the laser light source and is preferably in the kHz range.

9. Device according to one of claims 1 to 8, characterized ge indicates that between the external resonator (Ml, M2, M3) and laser light source (LD) in a known manner, preferably a piezoelectric (PZT2) adjustable mirror (3) for adjustment or modulation of the feedback phase is arranged.

10. Device according to one of claims 1 to 9, characterized ge indicates that the external resonator is a V-shaped Fox Smith resonator (M1, M2, M3), in which only that

Folding mirror (coupling mirror M2) passing through internal field is fed back into the laser light source (LD).

11. The device according to claim 10, characterized in that the distance between the resonator mirror (M3), which is not perpendicular to the light coupling direction through the folding mirror (M2), and the folding mirror equal to the distance from the folding mirror (M2) to the laser light source (LD ) is.

12. Device according to one of claims 1 to 11, characterized ge indicates that the detector device (PD1, AMP2, FIl, DBM2, PS2, FD) for detecting the component of the modu¬ oscillating with the double modulation frequency (2f _mo ) gated light is arranged in the beam path of the light reflected from the external resonator (M1, M2, M3) from the laser light source (LD).

13. Device according to one of claims 1 to 11, characterized ge indicates that the control device (PD1, AMP1, DBM1, PSl, INTl, SR) for controlling at least one operating parameter of the laser light source (LD) is a detector device (PD1, AMP1 , DBM1, PSl) for the detection of an amplitude modulation with the simple modulation frequency (f _mo ά in ^{the vo} external resonator (M1, M2, M3) reflected light, this detector means one of the modulation stroke and the phase position of the said amplitude modulation with the simple modulation frequency (f _{mo (j} ) dependent control signal (A) for controlling at least one operating parameter of the laser light source (LD).

14. Device according to one of claims 1 to 12, characterized in that the resonance center frequency (f _R ) of the exter¬ NEN resonator (Ml, M2, M3) is tunable.

15. The device according to claim 14, characterized in that a mirror of the resonator is piezoelectrically adjustable.

16. The device according to claim 14 or 15, characterized gekennzeich¬ net that the tuning range is in the order of 100 GHz and above.

17. Device according to one of claims 14 to 16, characterized ge indicates that a device (G, TR) for synchronously changing the resonance center frequency (f _R ) of the external resonator (M1, M2, M3) on the one hand and at least one of the the frequency (f _F ) of the free-running laser light source (LD) changing operating parameters is provided on the other hand.

18. Device according to claim 17, characterized in that the laser light source is a laser diode (LD) and the tuned operating parameter is the temperature of the laser diode (LD), for example one or more Peltier elements (6).

19. Device according to one of claims 14 to 18, characterized in that the tuning of the resonance center frequency takes place substantially linearly over time.

20. Device according to claim 19, characterized in that the tuning speed is in the order of a few GHz per second and above.

21. Device according to one of claims 1 to 20, characterized in that in addition to the external resonator (M1, M2, M3) outside the resonator of the laser light source (LD) an external reflector (MS) acting as a mode selector is arranged, which reflects at least part of the light back into the laser resonator (FIG. 4).

22. The device according to claim 21, characterized in that the distance of the external reflector (MS) from the laser resonator (LD), preferably by a reflector (MS) mounted on a piezo element (PZT), is adjustable.

23. The device according to claim 22, characterized in that the external reflector (MS) is partially transparent and a control device (LIA, 0P1, 0P2, PZT) is provided, which is a function of the light transmitted by the reflector (MS) the distance to the laser light source (LD).

24. Device according to claim 23, characterized in that a reflector modulation device (0P2, PZT) is provided for periodically modulating the reflector distance from the laser light source and that furthermore a phase-sensitive rectifying device (LIA) for determining a control signal for Regulation of the reflector distance from the laser light source from the photocurrent of the light transmitted by the reflector (MS) is provided.

25. The device according to claim 24, characterized in that the reflector modulation frequency is of the order of about 100 Hz.

26. Device according to claim 24 or 25, characterized gekennzeich¬ net that a device (CC) for modulating at least ei¬ nes operating parameter of the laser light source (LD) is provided with the reflector modulation frequency by which modulated by the distance from the laser light source To compensate reflector (MS) induced frequency modulation in the laser light source.