EP2481134A1

EP2481134A1 - Stabilisation of the repetition rate of a passively q-switched laser by means of coupled resonators

Info

Publication number: EP2481134A1
Application number: EP10790828A
Authority: EP
Inventors: Andreas TÜNNERMANN; Dirk Nodop; Alexander Steinmetz; Jens Limpert
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; Friedrich Schiller Universtaet Jena FSU
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; Friedrich Schiller Universtaet Jena FSU
Priority date: 2009-09-21
Filing date: 2010-09-17
Publication date: 2012-08-01
Also published as: DE102009042003A1; US20120242973A1; US8625644B2; WO2011032711A1; DE102009042003B4

Abstract

The invention relates to a Q-switched laser comprising a pump light source (1), an optical resonator, in which a laser medium (6) is situated and a passive Q-switch (5). The aim of the invention is to provide an improved Q-switched laser which has a simple, compact construction with a repetition rate that has at the same time the smallest possible degree of time jitter. To achieve this, according to the invention, part of the light that is decoupled from the optical resonator is fed to an optical delay line (9) by means of a beam splitter (8) and is re-coupled to the optical resonator once it has passed through said optical delay line (9). The optical resonator has a decoupling mirror (4) and the coupled resonator can contain a dichroic beam splitter (7) and a polarisation beam splitter (8).

Description

STABILIZING THE REPETITION RATE OF A PASSIVALLY SWITCHED LASER OVER

COUPLED RESONATORS

The invention relates to a Q-switched laser with a pump light source, an optical resonator in which a laser medium is located, and a passive Q-switch.

Q-switching is a commonly used technique for generating intense short pulses of light with lasers Q-switched lasers are versatile, with some applications focusing on short pulse duration, while other applications tend to focus on high pulse energy and peak power. The pulses generated by Q-switching typically take several tens of picoseconds to several hundred nanoseconds, and the pulse energy varies between a few nanojoules and many millijoules.

The principle of Q-switching is based on the fact that in a first phase, a certain amount of energy is stored in the laser medium by means of the pump light source and this energy is then retrieved in the form of a short pulse in a second phase. In the first phase, the laser action is prevented by the Q-switch in the optical resonator of the laser. After switching the Q-switch these losses are suddenly reduced. The fluorescent light emitted by the laser medium is now significantly amplified at each Resonatorumlauf. The gain is orders of magnitude higher than in the case of continuous operation. In this case, the optical power in the resonator and, accordingly, the power coupled out of the resonator increase over a plurality of resonator revolutions until the

BßgTÄTieU QS OPIE Light pulse the laser medium has taken a considerable part of the energy stored there. After that, the laser power drops sharply again, and a new cycle can begin.

The pumping light source of a Q-switched laser can be operated pulsed or continuously, which is often the case with diode lasers. The laser medium in Q-switched lasers must have the ability to store a significant amount of energy over a period of time.

In passive Q-switching, the Q-switch is a saturable absorber. This initially generates high losses, which however are overcompensated by the laser gain as soon as sufficient energy is stored in the laser medium. As soon as the laser power reaches a certain value, the absorption is strongly saturated, so that suddenly the gain increases and the power rises very fast, until the laser medium, in turn, a large part of the stored energy is withdrawn and the power decreases again. The saturable absorber functions as an automatically operated switch. The pulse repetition rate of a Q-switched laser is determined by the pump power, the saturable absorption, the effective number of laser ions involved, and other parameters. By suitable selection of the laser medium and the setting of various laser parameters, the pulse parameters of Q-switched lasers, in particular pulse duration, pulse energy and pulse repetition rate, can be varied within very large ranges.

Particularly short pulse durations can be achieved with microchip lasers, which are characterized by very short resonators without air gaps. These are characterized by a sandwich-like construction, which consists of a Auskoppelspiegel and a saturable absorber mirror (SESAM), which define the resonator of the laser, between the laser mirror and the saturable absorber mirror is the laser medium (eg a Nd: YVO crystal) , Such a structure is inexpensive to produce and also very compact. The pump light source is a laser diode whose light is coupled by a simple optics in the above-described resonator. The pulses generated in the resonator are separated from the pumping light by means of a dichroic mirror.

As explained above, in a passively Q-switched laser, the emission of the laser pulse is initiated by spontaneous emission as soon as sufficient energy is stored in the laser medium. Since spontaneous emission is a statistical process, the time between sufficient energy storage in the laser medium and initiation of the laser pulse varies. This fluctuation is called temporal jitter in passively Q-switched lasers. The jitter represents the mean fluctuation of the reciprocal of the pulse repetition rate of the laser. The jitter of passively Q-switched lasers is usually several orders of magnitude longer than the pulse duration. Disadvantageously, this problem excludes passively Q-switched lasers for all applications in which time synchronization with other processes is important.

Approaches are known from the prior art which reduce the jitter of a passively Q-switched laser.

On the one hand, a pulsed laser diode can be used as a pump light source. Due to the high pumping rate, the rate of spontaneous emission increases. This also increases the probability of initiation of the laser pulse. As a result, the jitter can be reduced by an order of magnitude, but it is still much larger than the pulse duration.

On the other hand, an external, pulsed laser can be used which at least partially saturates the saturable absorber or injects photons abruptly into the laser just before the passively Q-switched laser reaches the threshold of sufficient energy storage. Since the saturation of the absorber or feeding of photons into the resonator by means of the external laser happens at a defined time, the laser pulse is triggered at a defined time. In this way, the jitter can become smaller than the pulse duration. However, the external laser itself must deliver short pulses of sufficient power, which places high demands on the external laser. Against this background, it is an object of the invention to provide a simple and compact passively Q-switched laser that emits laser pulses with low temporal jitter.

This object is achieved by the invention on the basis of a passive Q-switched laser of the type specified above in that a part of the light coupled out of the optical resonator is fed to an optical delay path by means of a beam splitter and fed back into the optical resonator after passing through the optical delay path.

As explained above, in conventional passive Q-switched lasers, spontaneous emission photons initiate the laser pulse. This statistical process is, according to the invention, replaced by a deterministic process to reduce the fluctuations in the pulse repetition rate, i. minimize the time jitter. For this purpose, according to the invention, a portion of the generated laser pulse after coupling out of the resonator by means of a beam splitter is passed over a delay line and then fed back into the laser. The time delay generated by the delay line should be slightly smaller than the reciprocal of the pulse repetition rate of the laser so that sufficient energy can be stored to generate the next pulse by optical pumping in the laser medium. At the same time the laser pulse must not have been triggered by spontaneous emission at the time of the arrival of the returned light. The feedback laser pulse injects photons to initiate the subsequent light pulse and additionally causes (partial) saturation of the absorber, which also contributes to the initiation of the next light pulse. Both occur at a defined time, which is determined by the optical delay path, so that overall the temporal jitter can be reduced to the magnitude of the pulse duration itself.

According to a preferred embodiment of the laser according to the invention, the delay line is formed reflecting. This results in a particularly simple and compact design. The means of the beam splitter the Delay path supplied light pulse passes through this twice, namely in the forward and reverse directions.

As a delay path, a glass fiber is preferably used in the laser according to the invention, in which a part of the pulse energy is coupled and reflected back via a fiber Bragg grating at the end of the fiber in the laser resonator. The required fiber length I is thus I <c / 2nf _rep . Where n is the refractive index of the fiber material and f _{rep is} the pulse repetition rate of the laser. At a pulse repetition rate of, for example, 200 kHz, the fiber length must therefore be 500 meters, which is technically easy to implement. If the pulse repetition rate is increased by increasing the pump power, the laser will also _switch to jitter-reduced operation at integer multiples of the pulse repetition rate f _rep , since photons are then always made available at the right time for initiating the next laser pulse. That way, the laser can be up to 500 meters long

Delay line pulse repetition rates of 200, 400 1000kHz generate at a reduced jitter. The delay line can in principle also be realized by a conventional multipass cell made of mirrors.

An advantageous development is to provide two or more fiber Bragg gratings along the longitudinal extent of the light-conducting fiber. As a result, delay lines of different lengths are formed in each case. By suitably setting the pump power of the pump light source, the pulse repetition rate of the laser can then be adjusted so that the above-mentioned operation I <c / 2nf _{rep is} satisfied, ie the reciprocal of the pulse repetition rate of the free-running laser is somewhat greater than predetermined by the delay path. When coupling a portion of the decoupled from the optical resonator light in the optical delay line by means of the beam splitter, the laser then adjusts to the pulse repetition rate of the delay line, for which the above condition is best met. In this way, with a prefabricated delay line several different pulse repetition rates can be realized, in which the temporal jitter, as explained above, is reduced. Also advantageous is an embodiment of the laser according to the invention, in which the delay line is adjustable. This makes it possible to continuously tune the pulse repetition rate of the laser.

A useful development of the laser according to the invention is that the part of the light supplied by means of the beam splitter of the delay path passes through a frequency converter before being fed back into the optical resonator. In this embodiment, a part of the light fed back for the initiation of the next laser pulse has a different wavelength than the light of the generated light pulse and can be used for other applications.

According to a preferred embodiment of the invention, the optical resonator is followed by an optical amplifier. This may be e.g. to act a fiber amplifier in a conventional manner. Through the downstream optical amplifier, the power of the laser can be adjusted according to the application.

The laser according to the invention is suitable for use in distance measurement (LIDAR). Likewise, the laser is well suited for high-precision material processing. Further fields of application arise in the area of nonlinear frequency conversion and time-resolved spectroscopy.

An embodiment of the invention will be explained in more detail below with reference to FIGS. Show it:

Figure 1: schematic representation of the structure of the laser according to the invention;

FIG. 2 shows the time course of power, stored energy and losses in the laser according to the invention;

Figure 3: Representation of the temporal jitter of

Pulse repetition rate of a passive Q-switched laser without feedback according to the invention;

FIG. 4 shows a reduction of the temporal jitter according to the invention with feedback;

FIG. 5 depiction of the invention

Lasers as a block diagram.

FIG. 1 shows schematically the structure of the laser according to the invention. The light of a pump light source 1, which is e.g. is a laser diode is supplied via an existing two collector lenses 2, 3 simple optics an optical resonator, which consists of a Auskoppelspiegel 4 and a saturable absorber mirror 5. Between the Auskoppelspiegel 4 and the saturable absorber mirror 5 is a laser medium 6, which is optically pumped by the pump light source 1. The laser medium 6 is pumped by the pumping light source 1 until the inversion, i. the stored energy in the laser medium 6 provides sufficient optical gain to compensate for the losses of the saturable absorber mirror 5 and the losses resulting from the out-coupled light from the resonator. The laser then reaches its threshold. The laser light produced in the resonator 4, 5, 6 bleaches the saturable absorber 5, the quality of the resonator increases and a laser pulse is generated. The laser pulse is coupled out of the resonator and separated by a dichroic 7 from the pump light. After that, the process starts again. A portion of the decoupled laser pulse is fed back by means of a beam splitter 8 via a reflective delay line 9 in the laser. The delay time is slightly smaller than the reciprocal of the pulse repetition rate of the passively Q-switched laser. In the illustrated embodiment, the delay line 9 consists of a light-conducting fiber, the end having a reflective fiber Bragg grating.

FIG. 2 shows the time course of power 10, energy stored in the laser medium, ie inversion 11 and losses 12 in the laser illustrated in FIG. The drawn on the time axis arrow 13 marks the time of arrival of the means of the delay line 9 in time delayed feedback laser pulse. At the time 13, sufficient inversion 11 has already been stored to generate the next pulse by optically pumping the laser medium 6. The laser pulse has not yet been triggered by spontaneous emission. The feedback laser pulse injects photons to initiate the subsequent laser pulse 10 by stimulated emission. In addition, the feedback laser pulse causes (partially) sudden saturation of the saturable absorber 5 at the time 13, which also contributes to the initiation of the subsequent laser pulse 10. As can be seen in the diagram in FIG. 2, the inversion 11 decreases to a minimum during the generation of the laser pulse 10. After generation of the laser pulse 10, the losses 12 rise again in the laser resonator. After that, the process starts again. The feedback according to the invention reduces the temporal jitter of the pulse repetition to the order of magnitude of the pulse duration itself.

FIG. 3 illustrates the temporal jitter of the pulse repetition rate of the passively Q-switched laser shown in FIG. 1 without feedback according to the invention on the basis of an oscillogram. The laser pulse 14 shown in the middle of the diagram serves as a reference pulse. The arrows 15 illustrate the temporal variation, i. the jitter, in the generation of the subsequent laser pulse 16th

The oscillogram in FIG. 4 correspondingly shows the reduction of the temporal jitter in the case of the passive Q-switched laser with feedback according to the invention. In the oscillogram shown in FIG. 4, the temporal jitter is no longer visible. The jitter is reduced to the order of magnitude of the pulse duration itself.

According to the block diagram in FIG. 5, the laser according to the invention comprises an optical pump (with associated optics) 17. This pumps a passively Q-switched laser 18 of a type known per se. This has a repetition rate f _rep . A part of the light coupled out of the resonator of the laser 18 is fed to an optical delay line 19 and fed back after passing through the delay line 19. The time delay produced by the delay section 19 is At <n / f _rep. Where n is a natural number. At the time of the arrival of the feedback laser pulse, sufficient energy to generate the next pulse by optical pumping by the pump 17 must be stored in the laser medium of the laser 18. At the same time, at the time of the As a result, no new laser pulse has been triggered by spontaneous emission, and the system shown emits a jitter-reduced pulse train 20 as a result.

- Claims -

Claims

claims

1. Q-switched laser with a pump light source (1), an optical resonator in which a laser medium (6) is located, and a passive Q-switch (5),

In that a part of the light coupled out of the optical resonator is fed to an optical delay path (9) by means of a beam splitter (8) and is fed back into the optical resonator after passing through the optical delay path (9).

2. A laser according to claim 1, characterized in that the delay line (9) is designed such that the time delay with which the light is fed back into the resonator, slightly smaller than the reciprocal of the pulse repetition rate or slightly smaller than integer multiples of the reciprocal the pulse repetition rate of the laser is.

3. Laser according to claim 1 or 2, characterized in that the delay line (9) is reflective.

4. A laser according to claim 1 or 2, characterized in that the beam splitter (8) is a polarization beam splitter, a dielectric mirror or a fiber optic beam splitter.

5. A laser according to claim 4, characterized in that the delay line (9) comprises a light-conducting fiber, wherein the light-conducting fiber end has a reflective fiber Bragg grating.

6. Laser according to claim 5, characterized in that the light-conducting fiber has along its longitudinal extent two or more fiber Bragg gratings.

7. A laser according to claim 3, 4, 5 or 6, characterized in that the delay line (9) comprises a multipass cell with two or more mirrors.

8. Laser according to one of claims 1 to 7, characterized in that the pump power of the pump light source (1) is adjustable.

9. Laser according to one of claims 1 to 8, characterized in that the delay line (9) is adjustable.

10. Laser according to one of claims 1 to 9, characterized in that the means of the beam splitter (8) of the delay line (9) supplied part of the light before the feedback in the optical resonator passes through a frequency converter.

11. Laser according to one of claims 1 to 10, characterized in that the Q-switch (5) is a saturable absorber.

12. Laser according to one of claims 1 to 11, characterized by a resonator downstream optical amplifier.

13. Use of a laser according to one of claims 1 to 11 for the distance measurement, the material processing, the non-linear frequency conversion or time-resolved spectroscopy.

- Summary -