The performance of the passively Q-switched (PQS) laser deteriorates under high pumping power for the intracavity thermally induced wavefront distortion (thermal distortion for short). A new intracavity deformable mirror (DM) is proposed to compensate the thermal distortion of a PQS laser in this paper. The thermal distortion of the PQS laser is measured using the active deflectometry method. A simulation model is built to investigate the influences of the DM structure parameters on the surface shape of the DM (DMSS). Simulation results indicate that the DMSS matches well with the measured thermal distortion in the PQS laser at the given pumping current. Based on the simulation results, a low-cost, compact intracavity DM consisting of a mirror unit, a heater unit, a cooler unit and a base unit is built and used in the PQS laser. The DMSS is measured by a Zygo interferometer and coincides with the simulation result. In the improved PQS laser experiment, the optimum heater temperatures for the maximum output power and minimum M2 at different pumping currents are measured and given. The output stability of the PQS laser with the DM is tested. By adjusting the heater temperature, the PQS laser could achieve optimum performance in different environmental temperatures with good temperature adaptability. Experiment results verify that the PQS laser with the designed DM could achieve high output power and good beam quality at high pumping currents, as the DM prominently compensates the thermal distortion in the laser.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Owing to the short pulse duration and the high peak power [1,2], the PQS laser plays an important role in many fields, such as the remote sensing [3,4], the engine ignition [5,6], the micro-machining  and the optical parametric oscillators for generating mid-infrared laser pulses . However, in the high power pumping operation, the performance of the solid-state lasers will be limited by the thermal effects, e.g. the thermal lensing , which may degenerate the beam quality , lower the output power , change the cavity stability and even result in a complete termination of oscillation .
In order to compensate the thermal effect, a variety of intracavity DMs have been developed as useful solutions, including the piezoelectric (PZT) DM [13–19], the micro-machined DM (MMDM) [20, 21] and the pneumatic actuation DM [11,22,23]. However, as the PQS laser is short in cavity length, small in laser mode diameter and large in thermal load , these DMs have certain limitations when applied to the PQS laser. Generally, the PZT DM has a relatively large active aperture (e.g. >10mm). An intracavity beam expender [13–15] or a large cavity length (typically >1m) [16–19] is often needed in a laser cavity to generate a large laser mode diameter which matches with the large aperture of the PZT DM. Unfortunately, in the compact PQS laser, it is difficult to provide enough space for the large cavity length or other auxiliary elements. Recently, although some tailored PZT DMs could be specially set in miniature sizes (e.g. 3mm pitch), it is still quite difficult to directly match the active aperture of the DM with the small laser mode diameter (typically<1mm). The MMDM is an important choice to achieve the distortion compensation in the laser cavity for its small active aperture and numerous actuators. However, a MMDM with large stroke and high laser damage threshold for the PQS laser is usually high in price and could only be customized for most of the applications. Moreover, the control algorithm of a MMDM is complex. The pneumatic actuation DM, which has a small stroke (e.g. 1μm) and a large active aperture (e.g. several millimeters), is suitable for the distortion compensation in the thin disk lasers. Nevertheless, in consideration of the large thermal load and small mode size in the PQS laser, the pneumatic actuation DM is also inapplicable.
In this paper, a new intracavity DM is proposed for the compensation of the thermal distortion in the PQS laser. The single pass thermal distortion of a PQS laser is measured using the active deflectometry method [25–27]. A simulation model is built to investigate the influences of the DM structure parameters on the DMSS. The simulation results indicate that the surface shape of the designed DM at the heater temperature matches well with the measured thermal distortion in the PQS laser. Based on the simulation results, a low-cost, compact intracavity DM consisting of a mirror unit, a heater unit, a cooler unit and a base unit is built and used in the PQS laser. In the experiment, the optimum heater temperatures for the maximum output power and minimum M2 at different pumping currents are measured and given. The surface shape of the designed DM is measured and the output stability of the PQS laser with the working DM is tested. By adjusting the heater temperature, the PQS laser could achieve optimum performance in different environmental temperatures with good temperature adaptability. Experiment results verify that the PQS laser with the designed DM could achieve high output power and good beam quality at high pumping current, as the DM prominently compensates the thermal distortion in the laser.
2. Performance and thermal distortion of a PQS laser
2.1. Performance of a PQS laser
Figure 1 illustrates the schematic layout of the PQS laser built in our lab. A fiber coupled laser diode (LD) (808nm wavelength, 60W, 105μm core diameter, 0.22 NA, K808DA5RN-60.00W of BWT Beijing LTD) is mounted on a red copper block. A heating thermoelectric cooler (TEC) is set between the LD and the red copper block to control the LD temperature at 40°C. The coupling lenses (f30mm and f60mm, respectively) has a magnification of 7. The gain medium (1 in Fig. 1) is a 3mm × 3mm × 10mm 0.5 at. % Nd:YAG crystal, which is wrapped with indium foil and mounted in a red copper heat sink. Both of the red copper blocks (for the LD and the Nd:YAG crystal) are kept at a constant temperature of 25°C by the cooling water. A 0.8mm thick Cr:YAG crystal with the initial transmission of 83% at 1064nm is selected as the passive absorber (2 in Fig. 1). Mirror M1 is highly reflective (HR) at 808nm with an incident angle of 45° and highly transmitted (HT) for the visible light. Mirror M2 is the rear mirror (HR@1064nm, HT@808nm). Mirror M3 and M4 are intracavity 45° folding mirrors (HR@1064nm, HT@808nm). Mirror M5 is the output coupler (OC) with a transmission of 10%@1064nm. Mirror M1 to M5 are all plane mirrors. The total cavity length is 180mm. Mirror M6 and M7 are prisms used for the M2 measurement, which is achieved by a laser beam propagation analyzer (M2-200S-FW, Spiricon). Under these conditions, the passively Q-switched pulses with the wavelength of 1064nm and a pulse duration of 50ns (TDS 210 of Tektronix, 60MHz, 1GS/s) are generated.
Figure 2 shows the primary performance of the PQS laser with the increasing of the pumping current from 2.8A (the threshold current) to the target 3.9A. In Fig. 2(a), the pumping power and the continuous wave (CW) output power increase linearly with the pumping current to the maximum value of 19.20W and 5.48W, respectively. In Fig. 2(b), when the laser is in the pulse operation, the average output power achieves the maximum of 1.04W at 3.2A. As the pumping current increases, the gain increases accordingly. Meanwhile, the thermal lensing becomes serious, which could result in an unstable resonator. As a result of the comprehensive effect of the gain and the thermal lensing, the output reaches the maximum at 3.2A pumping current. When the current is smaller than 3.2A, the M2 is around 1.15 in both X and Y coordinates. The M2 starts to rise from 3.3A and reaches the maximum of 4.234 and 4.632 for the X and Y coordinates at the final 3.9A. The deterioration of the pulse performance from 3.3A to 3.9A is caused by the thermal effect in the cavity of the PQS laser. From Fig. 2(b), the strong thermal effect at high pumping current distorts the laser mode and results in the degeneration of the output power and the beam quality. In order to improve the pulse performance at higher pumping current, in our PQS laser setup, the mirror M4 will be replaced by an intracavity DM, which is designed to compensate the single pass thermal distortion of the fundamental mode.
2.2. Measurement and analysis of the thermal distortion
Some techniques have been developed for the measurement of the thermal distortion in the laser cavity, including the lateral-shear interferometric technique , the Shack–Hartmann wavefront sensor  and the active deflectometry method [25–27]. Considering the compactness, the convenience and the cost of the measurement setup, we choose the active deflectometry method to measure the single pass transmissive thermal distortion. The deflectometry system includes a liquid crystal display (LCD) (1024 × 768 pixels, UM-900 of Lilliput Co., Ltd), a CCD camera (1384 × 1036 pixels, GS3-U3-14S5M-C of Point Grey Research Inc.), a calibration board (3 in Fig. 1, comprising nine square distributed dots, with the dot diameter of 1.5mm and the distance between adjacent dots of 3mm), a filter (4 in Fig. 1, HR@808nm, HT@visible light wavelength) and a computer (not shown in Fig. 1).
In order to achieve high accuracy, the deflectometry system is calibrated before the measurement with the calibration board . Before the LD operating, two sets of grating pattern, with sinusoidal intensity profile along the X direction and the Y direction respectively, which are displayed on the LCD, transmit through the gain medium and image on the CCD camera. The measured results are applied as the reference patterns [Fig. 3(a), Fig. 3(c)]. When the PQS laser is operating, the refractive index of the gain medium is changed by the pumping light, which results in a phase distribution. The fringe patterns are distorted when transmitting through the gain medium and then carry the information of the phase distribution [Fig. 3(b), Fig. 3(d)]. After deducting the reference patterns from the distorted fringe patterns, a phase-shift algorithm is used to achieve the wavefront slope from the detected phase distributions, and a reconstruction algorithm is used to reconstruct the wavefront from the extracted wavefront slope . After the data processing in the computer, the thermal distortion could be obtained in high precision [Fig. 3(e)]. Note that the central light spots in Fig. 3(b) and Fig. 3(d) are filtered out in the data processing and will not affect the results of the thermal distortion. It also should be noted that the CCD camera in the measurement is focused onto the calibration board (the equivalent position to mirror M4) and the measured distortion (at the calibration board) could be considered equal to that at the position of mirror M4. Besides, only the central 1.5mm × 1.5mm area of the thermal distortion at M4 is picked out in the analysis [Fig. 3(e)], considering that the laser mode diameter is smaller than 1mm in the cavity and thus only the central thermal distortion affects the laser mode.
Figure 3(f) shows the one-dimensional distribution of the thermal distortion within the central 1.5mm length of mirror M4. The thermal distortion increases with the pumping current increasing from 3.3A to the target 3.9A. In Fig. 3(g), the peak to valley (PV) value of the wavefront distortion in the central 1.5mm × 1.5mm area of M4 increases linearly with the pumping current and achieves the maximal value (0.6850μm) at the final 3.9A.
In order to analyze the thermal distortion accurately, the two-dimensional thermal distortions at 3.3A and 3.9A are decomposed into the first 48 Zernike polynomials in Fig. 4. It can be seen that, the thermal distortions in this PQS laser mainly compose of the 3rd Zernike polynomial (defocus aberration, an amplitude of 0.4352μm and 0.6472μm at the 3.3A and 3.9A pumping current, respectively), while the higher order Zernike polynomials are minor and could be neglected (the amplitudes are smaller than 0.06μm or even much smaller). Based on the analysis result, it is inferred that if the 3rd Zernike polynomial of the thermal distortion could be compensated by the intracavity DM, and no large-amplitude high order Zernike polynomials are introduced, the performance of the PQS laser could be enhanced.
In consideration of the properties of the PQS laser and the limitations of the existing DMs (e.g. the PZT DM, the MMDM and the pneumatic actuation DM), it is important to develop a large stroked, low-cost and compact intracavity DM to achieve the thermal distortion compensation in the PQS laser.
3. Analysis and experiment of the Intracavity DM
3.1. Structure of the intracavity DM
In order to compensate the thermal distortion in the PQS laser, a special designed intracavity DM is presented in Fig. 5. The DM mainly consists of a mirror unit, a heater unit, a cooler unit and a base unit [Fig. 5(a)]. The first three units are screwed to the base unit. The mirror unit includes a BK7 mirror and a mirror holder. The diameter of the mirror is set to be 9mm to make the DM compact. The heater unit composes of a TEC (20mm × 20mm, 5V/2A, TES1-04802), a set of cooling fins and a heater rod, which are screwed together. The TEC is set between the heater rod and the cooling fins. The cooler unit consists of a cooling TEC (30mm × 30mm, 12V/3A, TEC1-11903), a water-cooled cooling block and a cooler rod. Note that each of the elements in the cooler unit has a through-hole in the center. The heater rod runs through the through-hole of the cooler unit and reaches the rear surface of the mirror. In order to provide strong strength and achieve the required surface shape, the heater rod is designed composing of two parts, i.e. a fore-end with a smaller diameter and a back-end with a larger diameter [Fig. 5(b)]. Accordingly, the cooler rod is divided into a fore-end (smaller diameter) and a back-end (larger diameter) as well [Fig. 5(b)]. In order to achieve fine heat conductivity, a thin film of silicon grease is applied on the rear surface of the mirror. The fore-end surfaces of the heater/cooler rods are gently pressed to touch the rear surface of the mirror, without any bonding. Two pieces of Negative Temperature Coefficient (NTC) thermistors (10KΩ) are bonded to the heater rod and the cooler rod respectively to monitor and display their temperatures. In the structure of the DM, the mirror holder, the heater rod, the cooler rod and the cooling fins and blocks are all made of Aluminum. The base unit includes a thermally insulated Polyetheretherketone (PEEK) base and an Aluminum base. Insulated by the PEEK, the heat is only conducted to the mirror through the heater/cooler rods.
3.2. Influences of the structure parameters
According to the structure shown in Fig. 5, the DMSS will be influenced by the length of the heater rod and the temperature distribution of the mirror. When the two TECs are in operation, heat transfers from the TECs to the heater rod and the cooler rod. The lengths of the two rods change accordingly. Meanwhile, the mirror center is heated and the outer annulus is cooled, which makes the mirror surface deforms. In order to investigate the influences of the structure parameters on the DMSS, a finite element model is built in ANSYS.
The model consists of a mirror, a heater rod and a cooler rod for simplicity. All the elements are bonded rigidly. The whole model is built with a 10-node tetrahedral structure solid element. The degree of freedom (DOF) of the heater rod bottom, the cooler rod bottom and the mirror side are set to zero. The initial surface shape of the mirror is plane. The air temperature is set 22°C. By means of natural convection air-cooling, the convective heat transfer coefficient is 10W/(m2K) on all the outer surfaces. All the material parameters in the simulation are listed in Table 1. The simulation results are shown in Fig. 6 (only the central 1.5mm × 1.5mm area of the DMSS is analyzed, the same size as the thermal distortion analyzed in section 2).
Figure 6(a) shows the PV value of the DMSS increases with the heater temperature, while Fig. 6(d) shows the DMSS rarely changes with the cooler temperature. Figure 6(b) shows that the maximum PV value appears at the 1.5mm diameter and the minimum PV value appears at the 3mm diameter, when the heater fore-end diameter increases from 1.5mm to 3mm with an increment of 0.5mm. Note that no flat-top surface shape appears in the center, even for the 3mm diameter. By contrast, a flat-top surface shape may appear in a PZT DM with such a large contact diameter (3mm) and such a thin mirror (e.g. 1mm). Figure 6(c) shows the PV value increases monotonically with the heater back-end length. In Fig. 6(e), when the inner diameter of the cooler fore-end increases from 2mm to 6mm with an increment of 1mm, the maximum PV value appears at the 4mm diameter. Figure 6(f) and Fig. 6(g) show that the PV value increases monotonically with the outer diameter of the cooler fore-end and the mirror diameter, respectively. In Fig. 6(h) and Fig. 6(i), the PV value decreases monotonically with the mirror thickness and the air temperature, respectively. From the analysis results, it could be concluded that the structure parameters could be optimized to ensure the deformed DMSS match the target surface shape.
3.3. Parameters of the designed DM and the simulation results
According to the performance of the PQS laser, the target surface shape is the thermal distortion at the 3.9A pumping current. The aim of the designed DM is to well compensate the distortion at the 3.9A pumping current. Meanwhile, the thermal distortion at other pumping currents could be mostly compensated as well. Using the design rules illustrated in section 3.2, the DM structure parameters are determined as shown in Table 2. The whole DM structure could achieve a compact size within 57mm × 42mm × 44.5mm, which is primarily restricted by the size of the TEC. If a smaller TEC (e.g. 10mm × 10mm, 5V/0.82A, TEC1-00705) is used, the structure of the DM could be much more compact.
Figure 7 shows the simulation results of the DMSS, setting the heater temperature 48°C, the cooler temperature 23°C and the air temperature 22°C. In Fig. 7(a), when the cooler is applied to the DM, the DMSS coincides well with the thermal distortion at the 3.9A pumping current [Fig. 3(f)]. By contrast, when the cooler is excluded, the PV value of the central surface shape is 30% smaller than that of the thermal distortion. In Fig. 7(b), when the cooler is included, the DMSS primarily consists of the 3rd Zernike polynomial whose Zernike coefficient is 0.6456μm and is almost equal to that of the thermal distortion at the 3.9A pumping current (i.e. 0.6472μm in Fig. 4). The coefficients of other Zernike polynomials (e.g. the 8th, the 15th, the 16th, the 27th and the 40th Zernike polynomials) are smaller than 0.06μm. Note that the 8th Zernike polynomial in the DMSS [Fig. 7(b)] partly offsets the same order of the thermal distortion [Fig. 4(b)]. By contrast, when the cooler is excluded [Fig. 7(c)], higher orders (e.g. the 8th, the 15th, the 16th, the 27th and the 40th Zernike polynomials) increase enormously in the negative direction. Figure 7(d) shows that, when the DM includes the cooler, the coefficient difference between the thermal distortion and the DMSS is much smaller than that of the case without the cooler. Based on these results, the cooler is optional for the design of the DM, depending on the PV values and the target surface shape. For instance, the cooler is indispensable for the distortion with large PV value (typically >2μm), while it may be no need for the distortion with small PV value (typically <2μm).
From Fig. 7, it could be concluded that the DMSS with a cooler matches well with the given thermal distortion. Moreover, by changing the structure parameters, the DM could compensate different thermal distortions in other solid-state lasers. It should also be worth to mention that, since the DM possesses only one actuator in the center, it could only compensate the circularly symmetric Zernike polynomials (e.g. the 3rd and the 8th Zernike polynomials), while almost has no impression on other Zernike polynomials (e.g. the 4th and the 5th Zernike polynomials).
3.4. Interferometer measurement results of the DM
Based on the simulation results above, a DM with the same parameters (Table. 2) is manufactured in our lab. The DMSS is measured using an interferometer (wavelength 632.8nm, 6〞VERIFIRE XPZ of Zygo Corp.), as shown in Fig. 8. To be comparable with the measured thermal distortions and the simulation results, only the central 1.5mm × 1.5mm area of the DMSS is analyzed.
According to Fig. 8, the PV value of the DMSS increases with the heater temperature [Fig. 8(a)]. When the heater temperature reaches 48°C (the same temperature as that set in the simulation), the DMSS almost matches with the thermal distortion at the 3.9A pumping current in Fig. 3(f), with the PV value only 0.02μm smaller than that of the thermal distortion. In Fig. 8(b), when the heater temperature is 48°C, the DMSS mainly consists of the 3rd Zernike polynomial whose Zernike coefficient (0.6910μm) is only 6.7% larger that of the thermal distortion at the 3.9A pumping current (0.6472μm in Fig. 4). The coefficients of other orders (e.g. the 8th, 15th, 16th, 27th and 40th order) are slightly smaller than the simulation results [Fig. 7(b)]. From Fig. 8, it could be concluded that the DMSS could compensate most of the target thermal distortion at the heater temperature of 48°C.
Furthermore, the linearity and the hysteresis of the DM are measured in the experiment [Fig. 9]. When the heating TEC is not in operation, the initial temperature is 22°C (the same as the air temperature). Afterwards, by adjusting the current of the heating TEC, the PV values of the DMSS are measured at five heater temperatures in both the heating and the cooling procedures. The PV value increases linearly from 0.1628μm to 0.6225μm, as the heater temperature rises from 22°C to 48°C. When the heater temperature drops from 48°C to 22°C, the PV value decreases linearly from 0.6225μm to 0.1617μm. The linearity is calculated to be 1.66% from the measurement results. The DM exhibits high linearity and almost no hysteresis, which makes it easy to realize close-loop operation.
By comparing Fig. 9 with Fig. 3(g), it could be seen that, the PV values of the DMSS in the heater temperature ranging from 38°C to 48°C match well with the thermal distortion PV values in the pumping current ranging from 3.3A to 3.9A. Thus, the performance of the PQS laser would be improved if the heater works in the temperature range mentioned above.
4. Improved PQS laser setup with an intracavity DM
In order to improve the output power and the beam quality, an intracavity DM presented in section 3 is introduced to the PQS laser in section 2.1 [Fig. 1], in which only the mirror M4 is replaced with the intracavity DM, with all the other setup parts being kept unchanged. The pulse duration of the PQS laser remains as 50ns. The experiment photos are shown in Fig. 10.
Figure 11 shows the experiment results of the average output power and the beam quality (M2) of the improved PQS laser. As the laser performance begins to deteriorate from the 3.3A pumping current (the hollow triangle marked curve in Fig. 11) with the standard mirror (mirror M4), the DM is controlled working when the pumping current is set above 3.3A. Figure 11 shows that, when the intracavity DM is in operation, the average output power increases monotonically and achieves a maximum of 1.15W at the 3.9A pumping current [the red solid triangle marked curve in Fig. 11(a)], while the M2 factor could stay stable around 1.24 and reaches a minimum of 1.127 at the 3.9A pumping current [the solid triangle marked curve in Fig. 11(b)]. These experiment results indicate that the performance of the PQS laser is greatly improved by using the intracavity DM. It is notable that the PQS laser remains compact after replacing the mirror M4 with the DM. From Fig. 1 and Fig. 10, comparing to the original straight cavity, the new Z-shape cavity is shorter in length and slightly wider in width.
Although the DM is optimized to mostly offset the thermal distortion at the 3.9A pumping current, the thermal distortions at other pumping currents could still be majorly compensated by changing the temperature of the heater, according to the analysis in section 3. The optimum heater temperatures (for the maximum output power and minimum M2) at different pumping currents are measured and given in Table. 3. It could be seen that, with the heater temperature increasing from 38.2°C to 48.1°C, the DMSS could well compensate the thermal distortion of the PQS laser from the 3.3A to the 3.9A pumping current. The experiment result in Table. 3 coincides well with the simulation result in Fig. 9.
Note that the optimum heater temperature could be influenced by the environmental temperature. Table 4 shows three sets of the optimum heater temperatures in different environmental temperatures. In our simulation and experiment, the environmental temperature is 22°C and the optimum heater temperature is 48°C. When the environmental temperature is 20°C or 24°C, the optimum heater temperature will change to 44°C or 52°C respectively. Thus, by adjusting the heater temperature, the PQS laser could achieve optimum performance in the practical environmental temperatures with good temperature adaptability.
Experiment results show that, if the practical heater temperature has a slight deviation (within about ± 1°C) from the optimum temperature, the performance of the PQS laser will not be obviously influenced. Take the 3.6A pumping current for an example [Fig. 12(a)]. The optimum heater temperature is 43.5°C for the 3.6A pumping current, with the maximum average power of 1.10W and the minimum M2 of 1.216. When the heater temperature varies between 42.5°C and 44°C, the average power remains larger than 1.00W, while the M2 is kept smaller than 1.350. The temperature range, within which the performance of the PQS laser could keep excellent, is defined as the heater temperature bandwidth. The heater temperature bandwidth enhances the adaptability of the PQS laser in the practical environment.
As shown in Fig. 12(b), the performance of the PQS laser in the range of 3.3A to 3.9A pumping current is measured, while the heater temperature is set 43.5°C (the optimum heater temperature for the 3.6A pumping current). It can be seen that, at the 3.7A pumping current, the PQS laser could also achieve high output power (1.11W) and good beam quality (M2 is 1.223 and 1.280 in the X and Y coordinates). From Table. 3, the optimum heater temperature is 45.1°C for the 3.7A pumping current. The 43.5°C is right within the temperature bandwidth of the 3.7A pumping current. Thus, the performance of the PQS laser could still keep improved at the 3.7A pumping current with 43.5°C heater temperature. For the optimum heater temperatures out of the temperature bandwidth, the performance of the PQS laser will get worse at the corresponding pumping currents. For instance, as the optimum heater temperature is 48.1°C for the 3.9A pumping current, the 43.5°C is far away from its bandwidth (4.6°C deviation). The PQS laser could only achieve the output power of 0.54W and M2 of 2.2. Actually, the essential reason of this phenomenon is that the DMSS at the 43.5°C heater temperature could not match with and compensate the thermal distortion at the 3.9A pumping current.
Figure 13 shows the M2 optimization procedure of the PQS laser at the 3.9A pumping current, when the DM is controlled working from a cold start. Before the DM starts working, the PQS operates in the stable state and the beam quality is very poor [M2 is 4.234 and 4.632 in the X and Y coordinates, Fig. 11(b)] with two spots existing in the laser beam [Fig. 13(I)]. When the DM starts working, the DMSS deforms accordingly from a plane to the target surface shape. It takes the DMSS about 200 seconds to reach the final stable state and match with the thermal distortion [Fig. 13(II)–13(IV)]. When the thermal distortion is compensated by the final DMSS, the laser beam is optimized to the fundamental Gaussian beam [Fig. 13(V)] and the beam quality (M2) is apparently improved to 1.127 and 1.148 in the X and the Y coordinates [Fig. 11(b)].
However, as the DM is actuated by the thermal conductivity, it suffers from a relatively long time to achieve the stable state between two temperatures with a large temperature interval. In practice, when the PQS laser is turned on from a cold start, the DM could be set working at the same time. Thus, when the PQS laser achieves the stable operation state, the DM could also complete the warm-up stage and reach the stable state.
During the operation of the PQS laser, when the pumping current of the PQS laser is changed, the switch time from one stable state to another may also be as long as few tens of seconds. If the PQS laser changes the pumping current from 3.6A to 3.8A, the switch time is about 15s. However, if the PQS laser changes the pumping current from 3.6A to 3.7A, the switch time is almost zero. The switch time greatly depends on the difference of the two pumping currents. Generally, the bigger difference, the longer switch time. Obviously, the presented DM is suitable for the quasi-static state applications, or the applications that do not need a high-speed bandwidth.
In our experiment, the output stability of the PQS laser with the working DM is tested at the 3.9A pumping current. Within one hour, the performance of the PQS laser is observed considerably stable [Fig. 14], since the DM and the laser have reached the stable state.
The designed DM is used to achieve the required surface shape in certain heater temperature to compensate the measured thermal distortion and improve the output power and the beam quality of the PQS laser. By adjusting the heater temperature, the required DMSS could be achieved in different environmental temperatures and the performance of the PQS laser could be improved effectively. Furthermore, as the DM exhibits high linearity of 1.66% and no hysteresis, it is easy to be used in the close-loop operation.
In conclusion, an intracavity DM is presented to compensate the large thermal distortion of a PQS laser. The thermal distortion of the PQS laser is measured using the active deflectometry method and decomposed into Zernike polynomials. A finite element model is built in the simulation, in which the influences of the structure parameters on the DMSS are simulated. The simulation results indicate that the surface shape of the designed DM at the heater temperature matches well with the measured thermal distortion in the PQS laser. According to the simulation results, a low-cost, compact intracavity DM consisting of a mirror unit, a heater unit, a cooler unit and a base unit is built. The measurement results of the DMSS by a Zygo interferometer coincide well with the simulation results. In the improved PQS laser experiment, a standard mirror is replaced by the intracavity DM. Experimental results show that the optimum heater temperature of the DM increases with the pumping current and the environmental temperature. In the experiment, the optimum heater temperatures at different pumping currents are measured and given. By adjusting the heater temperature, the PQS laser could achieve the optimum performance in different environmental temperatures with good temperature adaptability. Experiment results verify that the PQS laser with the designed DM could achieve high output power and good beam quality at high pumping currents, as the DM prominently compensates the thermal distortion in the laser.
National Natural Science Foundation of China (NSFC) (Grant No. 61775112).
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