Passively Q-switched ring laser

The principles of Q-switch lasers were described in detail in many books [134, 142, 156]. One of the major drawbacks of a passively Q-switched laser is the lack of a precise external trigger capability [156]. The pulse period is mainly determined by the photon dynamics in the cavity, and is vulnerable to environmental instabilities and spontaneous noise from the gain medium. Spontaneous noise can cause the pulse timing jitter in two ways. First, it perturbs the population difference of the saturable absorber, and second, a very small amount of spontaneous noise can take part in the Q-switching operation directly.

Figure 4.44 shows a typical passive Q-switched laser which operates at linear cavity without any external modulation. In general, the jitter as shown in Fig. 4.45 can range from 20% to almost 100% (i.e. no periodicity), depending on the pump power and/or the environmental instabilities [155].

Figure 4.44. A typical semi-symmetry linear cavity of passively Q-switched laser. The saturable absorber (Cr⁴⁺:YAG) is located next by the gain medium.

Figure 4.45. Timing jitter versus laser pumping power.

The inter pulse timing jitter of the repetitive period is usually less than 1%;

however, it increases to more than 20% for an integration over a few thousands of shots. In addition, the spatial hole-burning effect increases the instability for linear cavities. This jitter problem prevents the use of passively Q-switched lasers in many applications. Active stabilization of the timing jitter was demonstrated by external modulation of the population difference in the saturable absorber [156] and reduced the jitter down to 8%.

Here, another way to reduced jitter was proposed. The passive Q-switched laser is constructed by the novel two-mirror reentrant ring cavity as we mentioned in section 4.2. This unidirectional ring cavity geometry eliminates the effects of spontaneous noise from gain medium to the saturable absorber and spatial hole burning, which we will have a description later.

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Pumping Power (W)

Jitter (%)

As shown in Fig. 4.46, the system setup is the same as the figure “8” ring cavity in section 4.2. Passive Q-switching of the ring laser was achieved by inserting a Cr⁴⁺:YAG crystal of 90% low-power transmittance located at the opposite arm with respect to the gain medium; 90% low-power transmittance Cr⁴⁺:YAG was chose because the average output power is higher than 85% and 88% transmittance Cr⁴⁺:YAG from our experiments [156]. At this position, the Cr⁴⁺:YAG crystal stays at the beam waist to generate high peak power; in addition, there is minimum spontaneous noise from the gain medium. The saturable absorber is generally located close to the gain medium in a linear cavity; therefore, it is difficult to isolate the absorber from the spontaneous emission radiated from the gain medium. In our present setup, the thickness of the Cr⁴⁺:YAG crystal is about 0.8 mm. With an integration of over 52,000 shots, the timing jitter is maintained at less than 3% across a wide range of pumping powers in our non-planar figure “8” ring cavity.

Figure 4.46. The 3D views of the passively Q-switched ring laser cavity.

A. Laser performance

To optimize the laser performance under ring cavity operation, several output couplers with different transmittance were tested under a linear cavity condition with the same radius of curvatures as shown in Fig. 4.47.

Figure 4.47. L-I curves of different couplers transmittance.

The cavity mode size at Nd³⁺:YAG under the linear condition was intentionally adjusted to be the same as that of the ring cavity. As shown in Fig. 4.48, the round-trip mirror loss of 8% gives the best slope efficiency in the linear cavity.

Therefore, two spherical mirrors with 98% coating reflectance were adopted in the ring cavity configuration. This result of the transmittance serves as an optical coating design reference for better output power. As a result, an output peak power of 250W was achieved with a pump power of 1.26W as shown in Fig. 4.49. To optimized output power, different distances of the gain medium from cavity center were tried as shown in Fig. 4.50. From Fig. 4.50, it is found that the larger distance off the cavity

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98% & 94%

center, the smaller the output power; one of the reasons is that the large distance shift will introduce a smaller reflectivity of the laser mirror.

Figure 4.48. Slope efficiency of different round trip transmissions.

Figure 4.49. Peak power of the passively Q-switched ring cavity with the pulse width of 68 ns and round trip loss of the cavity is 8%.

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Slope efficiency (%)

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Peak power (W)

Figure 4.50. The output powers with gain medium in various off-axial distances.

B. Jitter reduction

Similarly, unidirectional operation was achieved by adding one intracavity reciprocal and one nonreciprocal polarization rotator in the nonplanar two-mirror ring cavity [118]. Monitoring with a Febry-Perot spectrometer, it is evident that the laser oscillates in a single longitudinal mode for all pump powers under unidirectional operation. The measurement set up of pulse jitter is shown in Fig. 4.51. An IR 1GHz low noise photo receiver is utilized, and the power supply is ±15 volt current limited.

Figure 4.52 shows the jitter under bidirectional and unidirectional operational operations with integrations over 52,000 pulses. In a typical linear Q-switched cavity, as the pumping power arises, the jitter should be decreased; that is because the spontaneous emission is less. In the ring cavity situation, the cavity configuration provided a mechanism that blocks most spontaneous emission, so that the difference

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d = 2 mm d = 3 mm d = 4 mm d = 5 mm

Pump power (mW)

Output power (mW)

of jitter reduction trend is not very obvious in unidirectional and bidirectional propagation.

Figure 4.53 depicts the pulse waveform, which has a width of 64 ns. The laser threshold is about 800mW. As the pumping power rises from 850 to 1300 mW, the repetition rate of the Q-switched ring laser under bidirectional and unidirectional operations range from 5 to 12 kHz and 6 to 16 kHz for high reflectivity (>99.8%) output couplers, and from 3.6 to 8.7 kHz and 2.5 to 7.5 kHz for 98% output couplers.

The jitter is significantly reduced in comparison with that in the linear cavity. Figure 4.54 shows the repetition rate of the same case, the repetition rate is proportional to the pumping power.

Figure 4.51. The jitter measurement set up.

Figure 4.52. The jitter measurement under unidirectional and bidirectional operations in the ring cavity. The square dots are bidirectional propagation and the jitter is around 6%, the circle dots are unidirectional propagation and the jitter is below 3.5% at various pumping powers.

Pump power (mW)

Timing jitter (%)

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bidirection

unidirection

Figure 4.53. The pulse waveform of the passively Q-switched ring cavity.

Figure 4.54. The repetition rate under unidirectional and bidirectional operations in the ring cavity. The square dots are for bidirectional propagation, the circle dots are for unidirectional propagation.

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Pump power (mW)

Repetition rate (kHz)

C. Summary

Usually, with external modulation on the population difference of the Cr⁴⁺:YAG crystal, the timing jitter can be reduced to be about 10% for linear cavities [154]. However, in the ring cavity scheme, even under bidirectional operation, the jitter can be reduced to below 8%. Remarkably, the timing jitter is further reduced to less than 4% for both 99.8 and 98% output couplers for all pump powers under unidirectional operation, where spatial-hole-burning-induced noise is eliminated. At certain pump power levels, the timing jitter can be as low as 1.5%.