## Investigations on acousto-optically Q-switched frequency-shifted feedback laser with Nd:YVO_{4} crystal

Optics Express, Vol. 15, Issue 12, pp. 7401-7406 (2007)

http://dx.doi.org/10.1364/OE.15.007401

Acrobat PDF (167 KB)

### Abstract

An acousto-optically Q-switched frequency-shifted feedback (FSF) diode-pumped laser with a Nd:YVO_{4} crystal is demonstrated. The laser emits more than 1W average power with multi-ten ns duration at up to 50 kHz repetition rate, and peak power higher than 1 kW is achieved at frequency of 20 kHz. This kind of pulsed laser has special value for some applications with outstanding features of broadband continuous spectrum and chirping frequency in the oscillation spectrum. Simulation results of the FSF laser through rate equations in Q-switched regime are also presented.

© 2007 Optical Society of America

## 1. Introduction

*et al*. reported a modelock-like pulsed output without Fabry–Perot frequency structure observed in 1988 [1

1. F. V. Kowalski, P. D. Halle, and S. J. Shattil, “Broadband continuous-wave laser,” Opt. Lett. **13**, 622–624 (1988). [CrossRef] [PubMed]

*et al*. and was called as modeless laser [2–4

2. I. C. M. Littler and J. H. Eschner, “The CW modeless laser model calculations of an active frequency-shifted feedback cavity,” Opt. Commun. **87**, 44–52 (1992). [CrossRef]

*et al*. conducted an experiment by using a Fabry-Perot interferometer with sufficiently low frequency-shifting rate (2.4×10

^{14}Hz/s), and it turned out that the output spectrum of a FSF laser consists of frequency components and the chirping is stepwise [5

5. S. Balle, I. C. M. Littler, K. Bergmann, and F. V. Kowalski, “Frequency-shifted feedback dye laser operating at a small shift frequency,” Opt. Commun. **102**, 166–174 (1993). [CrossRef]

_{4}FSF laser [6–8

6. K. Nakamura, K. Kasahara, M. Sato, and H. Ito, “Interferomeetric studies on a diode pumped Nd:YVO_{4} laser with frequency-shifted feedback,” Opt. Commun. **121**, 137–140 (1995). [CrossRef]

_{4}laser with an acousto-optical Q-switch (AOQ) and based on frequency-shifted feedback mechanism. No laser of this type has been constructed before, to our knowledge. Due to the prominent feature of its spectrum, that is, its broadband continuous spectrum and chirping frequency, this kind of pulsed laser is suitable as seed source for some laser amplifiers, especially a fiber one in which nonlinear process, such as stimulated Brillouin scattering (SBS), is prominent and harmful.

## 2. Experimental configuration

_{4}crystal pumped by a fiber-coupled diode was employed as the gain medium. Its dimensions were 3×3×5 mm and the doping concentration was 1.0 at. %; the left surface was coated with AR film at 808 nm and 1064 nm; and the right surface was coated with 808 nm HR film and 1064 nm AR film. The crystal was placed in a copper heat sink and a temperature controlling unit was used to control its temperature. The pumping LD, which was capable of emitting 10 W pumping light with wavelength of 808 nm, was coupled with a fiber bundle with total diameter of 400 μm and the numerical aperture of 0.37. Its working temperature was controlled at about 25°C by another temperature controlling unit. An AOM driven at frequency of 70 MHz was inserted into the cavity constructed by the full reflecting mirror M

_{2}and M

_{1}, which was coated with 1064 nm HR film and 808 nm HT film. The AOM had the maximal diffraction efficiency of about 75% and worked in Bragg diffraction mode. An AOQ was used to Q-switch the laser; its highest diffraction efficiency was 80% and the working frequency tuning range was 10k–150 kHz.

## 3. Experimental results

_{4}laser about 0.04 nm, that is 10 GHz in frequency.

## 4. Simulation results

### 4.1 Rate equations treatment

9. M. Stellpflug, G. Bonnet, B. W. Shore, and K. Bergmann, “Dynamics of frequency-shifted feedback lasers: simulation studies,” Opt. Express **11**, 2060–2080 (2003). [CrossRef] [PubMed]

*M*(q is an integer to identify each band) in a single band is given by

_{q}*c*is the light velocity in a vacuum;

*V*is the mode volume;

_{m}*σ*is the stimulated emission cross-section;

_{q}*l*is the constant loss except for useful output, such as absorption and diffraction loss,

_{c}*et al.*; and

*l*is the useful output. The total intracavity photon number is given by

_{o}*P*is the valid power absorbed by the gain medium,

_{a}*h*is the Planck constant,

*ν*is the photon frequency of pumping, and

_{p}*τ*is the lifetime of spontaneous emission.

_{f}*M*in band q with the photon number

_{q}*M*

_{q+1}in band q+1 after the roundtrip time. By solving the set of rate equations and the process of frequency shifting, we are able to obtain the intracavity photon number, the inversion number evolving with time, and the distribution of photon number on the spectrum. With the photon density in the cavity, the real-time power output is given by

*ν*is the frequency of the output laser. To compute the average of the real-time power over a period of time, we have the average power as

_{l}*t*is the calculating step time. The effective gain of the FSF laser is given by

*G*=

_{q}*B*-

_{q}Nτ_{R}*γ*, and the spectrum range of

*G*≤ 0 defines the spectrum width Δ

_{q}*ν*for the amplification process.

_{g}### 4.2 Q-switching results

### 5. Conclusions

_{4}crystal based on frequency-shifted feedback mechanism. The outstanding features of this kind of laser centralize on its spectrum. Frequency chirping is the intrinsic characteristic brought by the FSF mechanism; the chirping may be lethal for an optical process needing stable frequency, but it is significant for a situation in which a stable spectrum should be destroyed. For a quasi-continuous spectrum, all spectrum components in an output field may exist for a pulse with an appropriate duration. Taking the laser implemented in this work as an example, the chirping rate is 7×10

^{16}Hz/s, the longitudinal mode interval is 500 MHz, and for a pulse with a duration of 50 ns the chirping range is calculated as 3.5 GHz, which is much wider than a longitudinal mode interval. In addition, FSF mechanism exerts influence on the width of the output spectrum, which was measured as 0.11 nm or so and exhibits weak dependence on pumping level. The simulated results also confirmed these properties.

## References and links

1. | F. V. Kowalski, P. D. Halle, and S. J. Shattil, “Broadband continuous-wave laser,” Opt. Lett. |

2. | I. C. M. Littler and J. H. Eschner, “The CW modeless laser model calculations of an active frequency-shifted feedback cavity,” Opt. Commun. |

3. | I. C. M. Littler, S. Balk, and K. Bergmann, “The CW modeless laser: spectral control, performance data, and build-up dynamics,” Opt. Commun. |

4. | I. C. M. Littler, S. Balle, and K. Bergmann, “Continuous-wave laser without frequency-domain-mode structure: investigation of emission properties and buildup dynamics,” J. Opt. Soc. Am. B |

5. | S. Balle, I. C. M. Littler, K. Bergmann, and F. V. Kowalski, “Frequency-shifted feedback dye laser operating at a small shift frequency,” Opt. Commun. |

6. | K. Nakamura, K. Kasahara, M. Sato, and H. Ito, “Interferomeetric studies on a diode pumped Nd:YVO |

7. | K. Nakamura, F. Abe, K. Kasahara, T. Hara, M. Sato, and H. Ito, “Spectral characteristics of an all solid-state frequency-shifted feedback laser,” IEEE J. Quantum Electron. |

8. | K. Kasahara, K. Nakamura, M. Sato, and H. Ito, “Dynamic properties of an all solid-state frequency-shifted feedback laser,” IEEE J. Quantum Electron. |

9. | M. Stellpflug, G. Bonnet, B. W. Shore, and K. Bergmann, “Dynamics of frequency-shifted feedback lasers: simulation studies,” Opt. Express |

**OCIS Codes**

(140.3540) Lasers and laser optics : Lasers, Q-switched

(140.4780) Lasers and laser optics : Optical resonators

(190.2640) Nonlinear optics : Stimulated scattering, modulation, etc.

**ToC Category:**

Lasers and Laser Optics

**History**

Original Manuscript: March 6, 2007

Revised Manuscript: May 4, 2007

Manuscript Accepted: May 14, 2007

Published: June 1, 2007

**Citation**

Mali Gong, Chengqiang Lu, Ping Yan, and Huang Lei, "Investigations on acousto-optically Q-switched frequency-shifted feedback laser with Nd:YVO_{4} crystal," Opt. Express **15**, 7401-7406 (2007)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-12-7401

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### References

- F. V. Kowalski, P. D. Halle, and S. J. Shattil, "Broadband continuous-wave laser," Opt. Lett. 13, 622-624 (1988). [CrossRef] [PubMed]
- I. C. M. Littler and J. H. Eschner, "The CW modeless laser model calculations of an active frequency-shifted feedback cavity," Opt. Commun. 87, 44-52 (1992). [CrossRef]
- I. C. M. Littler, S. Balk, and K. Bergmann, "The CW modeless laser: spectral control, performance data, and build-up dynamics," Opt. Commun. 88, 514-522 (1992). [CrossRef]
- I. C. M. Littler, S. Balle, and K. Bergmann, "Continuous-wave laser without frequency-domain-mode structure: investigation of emission properties and buildup dynamics," J. Opt. Soc. Am. B 8, 1412-1420 (1991). [CrossRef]
- S. Balle, I. C. M. Littler, K. Bergmann, and F. V. Kowalski, "Frequency-shifted feedback dye laser operating at a small shift frequency," Opt. Commun. 102, 166-174 (1993). [CrossRef]
- K. Nakamura, K. Kasahara, M. Sato, and H. Ito, "Interferomeetric studies on a diode pumped Nd:YVO4 laser with frequency-shifted feedback," Opt. Commun. 121, 137-140 (1995). [CrossRef]
- K. Nakamura, F. Abe, K. Kasahara, T. Hara, M. Sato, and H. Ito, "Spectral characteristics of an all solid-state frequency-shifted feedback laser," IEEE J. Quantum Electron. 33, 103-111 (1997). [CrossRef]
- K. Kasahara, K. Nakamura, M. Sato, and H. Ito, "Dynamic properties of an all solid-state frequency-shifted feedback laser," IEEE J. Quantum Electron. 34, 190-203 (1998). [CrossRef]
- M. Stellpflug, G. Bonnet, B. W. Shore, and K. Bergmann, "Dynamics of frequency-shifted feedback lasers: simulation studies," Opt. Express 11, 2060-2080 (2003). [CrossRef] [PubMed]

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