Bidirectional operation of all-normal dispersion fiber lasers

Daojing Li

Supervisor: Deyuan Shen

Passive mode-locked fiber lasers have been extensively investigated in the past two decades because of their compactness, alignment-free operation and excellent pulse stability [1]–[7]. In recent years, dissipative soliton formed in the all-normal-dispersion fiber laser was reported, attracting researchers as it favors large pulse energy [2], [3]. Despite of different mode-locking techniques used, the multipulse operation always generated in the cavities [4].

Very recently, in the frame of complex cubic-quantic Ginzburg-Landau equation with certain parameter selections, the author of [5], [6] proposed a new concept of soliton formation, the so called “dissipative soliton resonance” (DSR).

In this case, the pulse peak power remains constant, while the pulse width could be arbitrarily broad and the pulse energy could be theoretically high at will. The DSR generation has been experimentally observed in mode-locked fiber lasers [7], [8].

The physical mechanism of the multipulse operation and the DSR generation in dissipative laser remains unclear, making it difficult to find the DSR generation regime in experiment. And for the DSR generation, the management of the generated pulse peak power was not yet studied.  In this work, we have numerically studied the mechanism of the multipulse operation and the DSR in an all-normal dispersion fiber ring mode-lock laser. It is showed that the spectral filtering effect, which limits the spectral maximum width, causes the multipulse operation in the dissipative soliton laser. Laser cavities with larger spectral filter bandwidth favor pulses with broader spectrum and higher peak power. To achieve the DSR generation in the cavity, strong peak-power-clamping effect of a sinusoidal SA is required. When the cavity peak-power-clamping effect is strong enough that the pulse.

References

[1]A. Grudinin, D. Richardson, and D. Payne, “Energy quantisation in figure eight fiber laser,” Electronics Letters, vol. 28, pp. 67–68(1), January 1992.

[2]L. M. Zhao, D. Y. Tang, and J. Wu, “Gain-guided soliton in a positive group-dispersion fiber laser,” Opt. Lett., vol. 31, no. 12, pp. 1788–1790, Jun 2006.

[3]A. Chong, W. H. Renninger, and F. W. Wise, “All-normal-dispersion femtosecond fiber laser with pulse energy above 20nj,” Opt. Lett., vol. 32, no. 16, pp. 2408–2410, Aug 2007.

[4]L. M. Zhao, D. Y. Tang, T. H. Cheng, H. Y. Tam, and C. Lu, “Generation of multiple gain-guided solitons in a fiber laser,” Opt. Lett., vol. 32, no. 11, pp. 1581–1583, Jun 2007.

[5]W. Chang, A. Ankiewicz, J. M. Soto-Crespo, and N. Akhmediev,“Dissipative soliton resonances,” Phys. Rev. A, vol. 78, p. 023830, Aug 2008. [Online]

[6]P. Grelu, W. Chang, A. Ankiewicz, J. M. Soto-Crespo, and N. Akhmediev, “Dissipative soliton resonance as a guideline for high-energy pulse laser oscillators,” J. Opt. Soc. Am. B, vol. 27, no. 11, pp. 2336–2341, Nov 2010.

[7]X. Wu, D. Y. Tang, H. Zhang, and L. M. Zhao, “Dissipative solitonbresonance in an all-normal-dispersionerbium-doped fiber laser,” Opt. Express, vol. 17, no. 7, pp. 5580–5584, Mar 2009.

Research progress of KDP crystal magnetorheological finishing

Student: Ji Fang

Supervisor: Xu Min

Potassium dihydrogen phosphate (KDP) is specialty crystal used in solid-state laser systems as optical frequency conversion and polarization electro-optical switching media. It is typical difficult material to processing due to its water solubility, low hardness, and temperature sensitivity. Today, KDP is finished via single-point diamond turning (SPDT) tools which are limited to surface corrections. Magnetorheological finishing (MRF) is a high-deterministic and flexible polishing technique developed in more than a decade. What is lacking is a usable nonaqueous MR fluid and finishing craftwork and subsequently cleaning technology.

The basic solvent and surfactant with both hydrophilic and hydrophobic groups are designed to assist the dispersion of inorganic particles in organic solvent. In order to maintain the ribbon and component steady during finishing, we develop the molecular bridging technology to connect solvent and particles and successfully prevent the solvent leakage. Small HF CIP is chosen as magnetic response particles according to material remove formula and core-shell composite structure is developed to soften particle’ surface hardness to lower the finishing roughness. Theoretic model for surfactant modification is proposed to assist the particles’ single dispersion and verified by experiment. The qualitative model for dispersion status and rheological property is proposed and verified via characterizing size distribution and viscosity.

In order to balance remove rate and scratch, we research and get the proper immersion depth of 0.1mm. The abrasive may easily float at the ribbon surface at high rotation speed under centrifugal force, so high speed would weaken the CIP finishing process and strengthen the abrasive finishing, which is beneficial for lowing roughness. CIP can be attracted and brought away by strong magnetic field easily and clean surface is obtained. We analyze the pathway influence on roughness and get the optimal mode. Circulation system is improved with conical container, large torque, and slender tube to adapt to the high viscosity slurry. Complex frequencies and blended reagents are explored based on the residual particles size distribution and the solvent chemical characteristic to achieve excellent cleaning effectiveness. The influence and acting mechanism of humidity to cleaning is analyzed.

Roughness is decreased and cutting waves are removed after finishing, and the figure accuracy correcting ability is better than that of LLNL. The PSD1 and residual force after finishing are obviously lowered. Though some achievements have been got, there are many problems need to be solved. We are working continuously towards to improve the finishing roughness, cleaning effectiveness to enhance the laser induced damage threshold (LIDT) of KDP.

Reference:

[1] B. L. Wang, H. Gao. Experimental Study on KDP Crystal Polishing. Proc. SPIE, 6722: 672209, 2007.

[2] S. D. Jacobs, S. R. Arrasmith, I. A. Kozhinova, et al. An overview of magnetorheological finishing (MRF) for precision optics manufacturing. Annual Meeting of the American Ceramic Society, 102, 1999..

[3] S. D. Jacobs, S. R. Arrasmith. Development of New Magnetorheological Fluids for Polishing CaF₂ and KDP. LLE Review, 80: 213-219, 1999.

[4] J. A. Menapace, P. R. Ehrmann, R. C. Bickel. Magnetorheological finishing (MRF) of potassium dihydrogen phosphate (KDP) crystals: nonaqueous fluids development, optical finish, and laser damage performance at 1064 nm and 532 nm. LLNL-PROC-420246, 2009.

[5] G. P. Tie, Y. F. Dai, C. L. Guan, et al. Research on subsurface defects of potassium dihydrogen phosphate crystals fabricated by single point diamond turning technique. Optical Engineering, 52(3): 033401, 2013.

High power and widely tunable Raman fiber lasers around 1.6 μm

Bihui Chen

Supervisor: Deyuan Shen

High power and tunable lasers operating in the eye-safe wavelength regime around 1.5-1.6μm have attracted great attention due to the wide applications in remote sensing, ranging and free space communications and as an efficient pump source for generating 3-5μm mid-infrared radiations via nonlinear frequency conversion. Compared to the traditional rare-earth-doped fiber lasers, Raman fiber lasers have the advantage of considerable flexibility in the operating wavelength selection through the choice of pump source and Raman gain fiber because of the broad Raman gain spectrum.

In this work, we have investigated high-power operation of fixed-wavelength and wavelength-tunable Raman fiber laser. In order to obtain the wavelength of 1658 nm, a home-constructed Er, Yb co-doped fiber laser wavelength-locked at 1545 nm is used as the pump source and it can offer a maximum output power of 36 W with a beam quality factor of M² ~3.2. The Raman gain fiber employed in this work is a standard graded-index multimode communication fiber with core diameter of 50μm ( 0.2 NA) and cladding diameter of 125μm.

For the fixed-wavelength Raman fiber laser, lasing characteristics of the free-running Raman fiber lasers for different gain fiber lengths are first evaluated. Under the optimum fiber length of 2.5km, a maximum output power of 12.3 W at ~1658 nm is generated for the launched pump power of 23.4W, corresponding to a slope efficiency of 86.3% with respect to the launched pump power. Then, a reflective VBG with a center wavelength of 1658 nm and spectral selectivity of 0.33nm is selected to lock the wavelength. We obtain output power up to 10.5 W at 1658.3 nm with a slope efficiency of 82.7% for the launched pump power of 23.4 W. The beam propagation factor (M²) measured under the maximum output power are ~1.35 and ~1.46 respectively. In terms of spectrum of the laser output, the VBG-locked output spectrum is much narrower with a FWHM bandwidth of ~0.1 nm compared to the FWHM bandwidth (~6 nm) of free-running operation. For the single-wavelength tunable Raman fiber laser, a 5.2 km fiber and another VBG with a center wavelength of 1750 nm and spectral selectivity of 0.8 nm are chosen to have good performance. A tuning range of 37 nm from 1638.5 to 1675.1 nm is obtained with a maximum output power of 3.6 W at 1658.5 nm for the launched pump power of 13.0 W. The spectral linewidth remained approximately constant over the whole tuning range at less than ~0.3 nm. For the dual-wavelength tunable Raman fiber laser, the single-path pump configuration in free-running operation is first done to determine the optimum Raman fiber length. The 6.5 km Raman fiber laser has the best performance concerning output power and slope efficiency. A maximum output power of 12.3 W is generated for the launched pump power of 21.5W. Based on the experimental and the simulation results, the 6.5 km Raman fiber is used as the gain fiber in the next experiment. To obtain dual-wavelength operation, two VBGs with centre wavelengths of 1658 nm and 1750 nm respectively are parallel-aligned to generate two wavelengths oscillation simultaneously. The wavelength tuning range is from 1621.3 nm to 1683.5 nm covering the whole U-band for the launched pump power of 17.5 W. The wavelength difference (Δλ) is tuned from 0.7 nm to 62.2 nm and the FWHM bandwidth of each individual wavelength is measured to be less than 0.35 nm.

References

[1]  J. Liu, D. Y. Shen, H. T. Huang, X. Q. Zhang, L. Wang, and D. Y. Fan, IEEE J. Quantum Electron. 2014, 50, 88.

[2]  F. Havermeyer, W. Liu and G. J. Steckman, Opt. Eng. 2004, 43, 2017.

[3] S. H. Baek and W. B. Roh, Opt. Letter. 2004, 29, 153.

[4] N. B. Terry, K. T. Engel, and T. H. Russell, Opt. Express. 2007, 15, 602.

[5]  J. Liu, D. Y. Shen, X. Q. Zhang, D. Y. Fan, Opt. Express. 2014, 22, 6605.

[6]  W. C. Yao, J. Liu, N. Y. Chen, J. N. Zhang, Y. G. Zhao, and D. Y. Shen, IEEE Photonics Journal. 2014.

Time: 6:30 pm, Thursday, 2014.12.11

Location: Optical Building. Room 525