WO1997031411A1

WO1997031411A1 - Stabilisation of a pulsed laser

Info

Publication number: WO1997031411A1
Application number: PCT/SE1997/000165
Authority: WO
Inventors: Magnus Arvidsson; Björn HANSSON; Carsten LINDSTRÖM
Original assignee: Geotronics AB
Current assignee: Geotronics AB
Priority date: 1996-02-20
Filing date: 1997-02-04
Publication date: 1997-08-28
Anticipated expiration: 1998-08-20
Also published as: SE9603288D0; SE9603288L

Abstract

The invention relates to a stabilization method and device for a passively Q-switched laser, which is pumped by at least one pump laser source (6, 6'; 12). The pump power of the pump laser source (6, 6'; 12) is modulated between a pump level lower than and a pump level above the lasing threshold for the Q-switched laser at frequencies which are always lower than a self-pulsation frequency of the Q-switched laser.

Description

Geotronics AB

Stabilisation of a pulsed laser

This invention relates to the stabilization of a pulsed laser of the kind apparent from the introductory part of claim 1, and to the stabilization of for instance a passively Q- switched micro-chip laser.

BACKGROUND OF THE INVENTION

Diode pumped solid-state lasers are a rapidly growing field. Passively Q-switched micro-chip lasers are particularly interesting as they are able to provide short pulses (< Ins) with high peak power (KW) for moderate pump powers, from, for instance, a laser diode or a Ti:Saphire laser or the like, using an extremely simple configuration.

A disadvantage with this kind of Q-switched laser has been that a passively Q-switched laser has a considerable pulse repetition frequency jitter accompanied by pulse-to-pulse amplitude fluctuation.

Actively Q-switched lasers, i.e. Q-switched lasers in which the control of the Q-switching is done directly at the Q- switch crystal, for instance by changing its polarization, are larger than passive Q-switched lasers and they also need fast high voltage switching power supplies to work.

There is a need for a passively Q-switched laser in a lot of applications, since they can be made smaller in size and do not require the high voltage switching devices normally required in actively Q-switched lasers. However, passively Q- switched lasers do need to be stabilized.

A passively Q-switched micro-chip laser for producing high- peak-power pulses of light of extremely short duration is described in US-A-5,394,413. A saturable absorber prevents the onset of lasing until the average inversion density within the cavity of the Q-switched laser reaches a predeter¬ mined value. The configuration of the laser is then such that, at the onset of lasing, a Q-switched output pulse having an extremely short length and a high peak power is generated.

This kind of extremely short duration, high-peak-power laser pulses could be useful in many applications, such as in electronic distance measuring devices (EDM). In this case, it is important to generate each short Q-switched laser pulse at an exactly determinable time. It is also important to generate a pulse train having exactly determinable times between the individual pulses, i.e. the jitter between the pulses should be kept at the lowest possible level.

OBJECTS OF THE INVENTION

An object of the invention is to provide stabilization of the output from a passively Q-switched laser, for instance a Q- switched micro-chip laser.

Another object of the invention is to provide stabilization of a Q-switched laser in such a way that a train of narrow pulses can be provided having as little jitter as possible between the output pulses.

Still another object of the invention is to provide stabiliz¬ ation of a passively Q-switched laser so that the time of the output of each laser pulse is exactly determinable.

These objects are achieved by a method having the features in claim 1, and a device for performing the method is disclosed in claim 7. Further features and improvements of the invent- ion, are disclosed in the dependent claims.

The invention relates to a stabilization method and device for a passively Q-switched laser, which is pumped by at least one pump laser source. The pump power of the pump laser source is modulated between a pump level lower than and a pump level above the lasing threshold for the Q-switched laser at frequencies, which are always lower than a self- pulsation frequency of the Q-switched laser. Each pump pulse having a pump level above the laser threshold is present at least until an output pulse from the Q-switched laser has been emitted. Preferably, the pulse length of each pump pulse is controlled by the output pulse from the Q-switched laser. The modulation of the pump laser source could comprise only ac power components. However, the modulation of the pump laser source could also comprise an ac modulation signal superimposed on a dc level. The modulation of the pump laser source could also be controlled to have a varying frequency during an operation period.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and for further objects and advantages thereof, reference is now made to the following description of examples of embodiments thereof, as shown in the accompanying drawings, in which:

FIG. 1 shows schematically a first embodiment of the Q- switched laser according to the invention; FIG. 2A and 2B shows schematically a first embodiment of a timing diagram for providing the stabilization according to the invention; FIG. 3 shows schematically the jitter varying with changing duty cycle; FIG. 4 shows schematically the jitter varying with changing frequency; FIG. 5A and 5B shows schematically a second embodiment of a timing diagram for providing the stabilization according to the invention; FIG. 6 shows schematically a second embodiment of the Q- switched laser according to the invention; FIG. 7A and 7B shows schematically a timing diagram which can be produced by the embodiment of the invention illu¬ strated in FIG. 6.

FIG. 1 shows a passively Q-switched laser 1, such as a solid- state Q-switched laser having a cavity comprising an active medium 2 of, for instance, Nd:YV0₄ crystal having a mirror 3 at one end and as the Q-switch crystal a saturable absorber 4 of, for instance, Cr*^*:YAG or Cr⁺:YSGG or the like at its other end. A second mirror 5 is placed at the output end of the Q-switched laser near to the saturable absorber 4. In the embodiment shown the mirror 5 is planoconcave and has a high reflectance. The Q-switched laser 1 could be a micro-chip laser.

The Q-switched laser is pumped by at least one laser diode 6 (and 6' ), which has a wavelength matched to the absorption band of the active material, through a lens system 7 or other optical means leading the light from the pump laser diode 6 to the gain medium 2 through the mirror 3. In the case of the lasing crystal being Nd ^*:YV0₄ the pump wavelength is about 808 nm. It is also possible to use more than one laser diode. The laser diode 6 (or diodes 6, 6') has power supplied by a pulse control device 8. The control device controls the pulses and can be adjusted so that the pulse length is adapted to the output of the solid-state laser pulses either by an operator or, preferably, a sensor 9 sensing the Q- switched laser output and connected to the control device 8 which turns off the control pulse to the laser diode as soon as the Q-switched laser output has been produced.

Some other kind of pump light source could be used instead of a laser diode, for instance a Ti:Saphire laser, which can be modulated by an acousto-optical modulator.

According to the invention time stabilization of the Q- switched laser output is provided by modulating the pumping and locking the frequency of the Q-switched laser to the pumping modulation. Thus, the pump power is momentarily increased a predetermined time before a desired Q-switched laser output pulse. If the energy in each pump pulse is adjusted such that the Q-switched laser only generates one pulse per pump pulse then frequency locking is achieved. The rest inversion in the laser cavity will decay during the time between the pump pulses, and thus the conditions in the laser cavity could be the same at the beginning of each pump pulse if the pause between pulses is long enough. It is thus possible to get a very stable repetition rate, and also a stable amplitude, from a Q-switched laser stabilized in this way. Thus, a stable Q-switched pulse train having adjustable pulse rate can be provided for frequences under the self- pulsation frequency f_βf provided from a constantly pumped Q- switch laser, i.e. without any pump modulation. Thus, the frequency region to be used for the pump power modulation is between 0 to f_sf for the Q-switched laser in question.

The diagram shown in FIGs 2A and 2B illustrates the preferred way to pump the Q-switched laser according to the invention. The control circuit 8 in this embodiment comprises a pulse oscillator and, as is apparant from FIG 2A, supplies pulses having a peak power and a duration providing an energy large enough for building up the critical value for providing the onset of the Q-switched laser pulse. As is apparent from FIG 2B the Q-switched laser pulse is generated just before the end of each pump pulse.

In an experiment the cavity consisted of a 1mm Nd:YV0₄ cryst- al 2, the passive Q-switch 4 was a Cr:YAG crystal with T=90% and the outcoupling mirror had a reflectance 90% and a con¬ cave radius of 50 mm. The cavity was pumped with 390 mW CW input power, giving a self pulsing frequency of 13 kHz and a pulse length 20 ns. The self-pulsing jitter was measured and found to be 60 ns. When applying modulation to the input light a jitter of 33 ps was produced at a frequency of 11.7 kHz. A 100% modulation was then used, i.e. going from zero effect up to full effect. The jitter varied as the duty cycle is changed, as is apparent from FIG 3. The current to the laser diode was adjusted so that the Q-switched laser was locked onto the pumping frequency while the pulse length was varied. The jitter from the Q-switched laser then varied from 33 ps at a 40% duty cycle to 200 ps at 80% duty cycle. Thus for large duty cycles there is less time for the rest inversion to decay, but it is apparent that for a duty cycle as large as 55% there is still a jitter as low as 70 ps.

In the experiment the pump power needed to lock the diode laser varied from 220 mW at 40% duty cycle to 290mW at 80% duty cycle. A more efficient pumping was achieved for the lower duty cycle due to the shorter time from the beginning of the pulse to the output of the Q-switched laser pulse. Less energy is lost by spontaneous emission before the pulse is built up at lower duty cycles. This effect became import¬ ant in the experimental device since the lifetime in ND:YV0₄ is 90μs.

The repetition rate of the Q-switched pulse train can be adjusted while still retaining a small jitter, as is apparent from FIG 4. The pump frequency was changed while the pulse width was kept constant at 43μs. The period could be adjusted from 84μs to 190μs producing a jitter of from 35ps to 120ps.

The experiment also showed that the amplitude jitter between the Q-switched laser pulses was negligable.

The best result was obtained for 100% modulation, i.e. a modulation comprising only ac components and thus not a dc level. However, the principle according to the invention is not limited to that. In some cases it could be convenient to have the Q-switched laser pumped with a constant light having a pump power level which is under the lasing threshold and to activate the Q-switched laser output pulse by increasing the pump power from that level. This kind of control is shown in FIG 5A, showing modulation pulses superimposed on a constant power level. This could for instance be achieved by having two laser diodes, one being supplied with a constant current and the other with a pulsed current. The time of the modulat- ion pulses are essentially shorter than in a case with 100% modulation. A stable pulse train can be obtained but the time jitter between the pulses is increased in relation to 100% modulation. The laser frequency could be increased up to the self-pulsation frequency without having to change the shape of the pump pulse.

The frequency could be varied within a Q-switched pulse train as long as it does not exceed the self-pulsation frequency and this means that it is possible to control the emittance of each Q-switched pulse in the train individually. In the embodiment shown in FIG 6, a Q-switched pulse is emitted to a target and reflected back to a pulse receiver 10. The receiv¬ er 10 controls a control circuit 11 which after a predeter¬ mined delay activates the laser diode 12. The Q-switched laser pulse is sensed by the sensor 13 connected to a reset input of the control circuit 11 which then inactivates the laser diode 12. The control circuit 11 also comprises means to make an extra delay of the control pulse to the diode 12 if the pulse received by the receiver should come so early that the momentary frequency should happen to coincide with or be above the self-pulsation frequency.

A timing diagram of the same kind as illustrated in the FIGs 2A and 2B but related to the embodiment shown in FIG. 6 is presented in FIGs 7A and 7B. It is apparent here that it is possible to have quite different pulse pauses between the individual pump pulses.

While the invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the accompany¬ ing claims. In addition, modifications may be made without departing from the essential teachings of the invention as defined in the claims. One example of such a variation would be to use the stabilization method on passively Q-switched lasers other than the one described in relation to FIG. 1. Thus, the stabilization method is not dependent upon the materials in and the exact design of the laser cavity.

Claims

1. Stabilization method for a passively Q-switched laser, which is pumped by at least one pump laser source (6, 6';12), characterized in that the pump power of the pump laser source (6, 6';12) is modulated between a pump level lower than and a pump level above the lasing threshold for the Q-switched laser at frequencies which are always lower than a self- pulsation frequency of the Q-switched laser.

2. Stabilization method according to claim 1, characterized in that each pump pulse having a pump level above the laser threshold is present at least until an output pulse from the Q-switched laser has been emitted.

3. Stabilization method according to claim 1 or 2, charac - erized in that the pulse length of each pump pulse is con¬ trolled by the output pulse from the Q-switched laser.

4. Stabilization method according to any of the preceding claims, characterized in that the modulation of the pump laser source comprises only ac power components.

5. Stabilization method according to anyone of the claims 1 to 3, characterized in that the modulation of the pump laser source comprises an ac modulation signal superimposed on a dc level.

6. Stabilization method according to anyone of the preceding claims, characterized in that the modulation of the pump laser source is controlled so as to have a varying frequency during an operation period.

7. Stabilization device for a passively Q-switched laser, which is pumped by at least one pump light source (6), char¬ acterized by a control circuit (8;12) modulating the pump power of the pump laser source (6,6';12) between a pump level lower than and a pump level above the lasing threshold for the Q-switched laser at frequencies which are always lower than a self-pulsation frequency of the Q-switched laser.

8. Stabilization device according to claim 7, characterized in that the control circuit (8;12) controls the pump laser source to provide pump pulses, each pump pulse having a pump level above the laser threshold and being present at least until an output pulse from the Q-switched laser has been emitted.

9. Stabilization device according to claim 7 or 8, charac¬ terized by a sensor (9;13) for sensing the appearance of the output pulse from the Q-switched laser and for resetting the control circuit (8;12) to turn off a pump pulse when an output pulse is sensed.

10. Stabilization device according to any of the claims 7 to 9, characterized in that the control circuit (8;9) modulates the pump laser source with a signal comprising only ac power components.

11. Stabilization device according to any of the claims 7 to 9, characterized in that the control circuit (8;9) modulates the pump laser source (6,6' ) with a signal comprising an ac modulation signal superimposed on a dc level.

12. Stabilization device according to any of the claims 7 to 11, characterized in that the control circuit (8;9) controls the modulation of the pump laser source to have a varying frequency during an operation period.

13. Stabilization device according to claim 12, characterized by a receiver (10) which controls the control circuit (11) whereby after a predetermined delay the control circuit activates the laser diode (12); and by the control circuit (11) also comprising means to make an extra delay of the control pulse to the diode (12) if the pulse received by the receiver should come so early that the momentary frequency should happen to coincide with or be above the self-pulsation frequency.