Yongdan Hu

Low-noise narrow-linewidth fiber laser at 1550 nm (June 2003)

We present a compact integrated fiber laser with more than 200 mW of output power. It combines polarized fiber output with very narrow linewidth of less than 2 kHz. The coherence length of the laser is measured to be longer than 5 km. The laser features high mode stability of less than /spl plusmn/10 MHz over hours. The relative intensity noise (RIN) spectrum is dominated by a peak at the relaxation oscillation frequency and is shot-noise limited otherwise. The RIN peak at 1 MHz is reduced to /spl sim/-130 dB/Hz by integrating a negative feedback circuit. In addition to thermal wavelength tuning, the laser frequency can be modulated at a bandwidth of up to 10 kHz via the piezoelectric effect.

Waveguide distributed Bragg reflector laser arrays in erbium doped glass made by dry Ag film ion exchange

We demonstrate a distributed Bragg reflector (DBR) waveguide laser array by using an Ag film ion-exchange technique. We achieve an output power of 11 mW at 1540 nm for a coupled pump power of 145 mW, with a threshold of 60 mW and slope efficiency of 13%. The thin film ion exchange produces the highest index change possible at the surface, due to the ion exchange technique. Hence a wide array of wavelengths can be implemented in a single chip. We demonstrate a lasing wavelength range of 2.1 nm (1548.6 to 1550.7 nm) in an array with a single grating. Since the index change due to our process is large, we can fine tune the wavelengths of the array to fall on International Telecommunication Union (ITU) grid wavelengths by annealing. We demonstrate fine tuning of the wavelength for a channel from 1540.2 to 1540 nm, ITU specified wavelength, by annealing.

Compact multimode pumped erbium-doped phosphate fiber amplifiers

The performance of compact multimode pumped erbium-doped phosphate fiber amplifiers is presented. A fiber amplifier with a small signal net gain of 41 dB at 1535 nm and 21 dB over the full C-band is demonstrated using a newly developed 8-cm-long erbium-doped phosphate fiber excited with a 1-W, 975-nm multimode laser diode. A theoretical model is developed for the multimode pumped amplifier based on modified rate equations and an effective beam propagation method. Close agreement between experimental and modeling results is observed.

Single-frequency, all-fiber Q-switched laser at 1550-nm

We present a novel actively Q-switched single frequency, linear, all-fiber laser using fast stress-induced birefringence modulation. We demonstrate 25-W peak power in 12-ns, 0.3-µJ pulses at a repetition rate of 80 kHz with 370 mW of pump power, without using an amplifier. Very effective Q-switching was also observed at 325 kHz, near the resonance frequency of the piezoelectric actuator. Using a higher pump power of 780 mW, an average output power of 85 mW and pulsewidth of 16 ns at 325 kHz PRF was observed.

Optical waveguides prepared in Er3+-doped tellurite glass by Ag+-Na+ ion exchange

This paper reports the preparation of planar waveguides in an Er3+ doped tellurite glass with a composition, 15Ns2O-25WO3-60TeO2- 1Er2O3, by Ag+-Na+ ion-exchange. The glass was chemically stable during ion-exchange process. Single-mode and multi- mode planar and buried waveguides in the tellurite glasses have been prepared, which is the first demonstrated of waveguides in tellurite glasses fabricated by ion-exchange technique to the best of our knowledge. The depths of the waveguides could be controlled by varying ion-exchange temperatures and times. The diffusion parameters, diffusion coefficients and activation energy were estimated. It is indicated that the tellurite glass is a prospective candidate for active integrated optics.

All-fiber Q-switched laser

An all-fiber Q-switched laser includes a gain fiber spliced between narrowband and broadband fiber gratings that define a polarization-dependent resonant cavity. The narrowband grating is, for example, formed in a PM fiber to create a polarization-dependent reflection band. A modulator applies stress to the fiber chain to induce birefringence and switch the cavity Q-factor to alternately store energy in the gain fiber and then release the energy in a laser pulse.An all fiber Q-switched laser with Mach-Zehnder Interferometer configuration has been reported. A fiber Bragg grating is used to achieve frequency-choice and tuning. The laser can generate stable optical pulse at center wavelength 156mm, peak power 2.6w, pulse width 4.2ps at repetition rate 760Hz, pumped by a laser diode 980nm. Self-mode locking in the Qswitched all fiber laser has been demonstrated. If changing period of fiber grating through adjusting stress or temperature, we can obtain continuous wavelength tunable output optical pulse and the tunable range is about 5rim.

Multi-mode pumped amplifier using a newly developed 8-cm long erbium-doped phosphate glass fiber

A fiber amplifier with a net gain of 43dB at 1535nm and 21dB over the full C-band was demonstrated using a newly developed 8-cm long erbium-doped phosphate fiber excited with a 1 W 975nm multimode laser diode. A theoretical model was developed for multi-mode pumped amplifier using a very short fiber.

Cooperative upconversion in a new high Er-doped tellurite glass

A model for population dynamics of Er3+ in tellurite glasses with high erbium concentrations is proposed. Luminescence decay curves from transitions of 4I13/2 yields 4I15/2 and 4S3/2 yields 4I15/2 were measured experimentally. Cooperative upconversion coefficients were obtained by numerically solving the rate equations of the model to fit the population dynamics of the 4I13/2 and 4S3/2 levels. The cooperative upconversion coefficient for 4I13/2 + 4I9/2 yields 4I15/2 + 4I15/2 is found to be 2.74 X 10-18 cm3/s and for 4I11/2 + 4I11/2 yields 4I15/2 + 2F7/2 is 1.09 X 10-18 cm3/s. The model provides a god basis for explaining the experiment data as it shows a high sensitivity to the input fitting parameters. This model can be used to analyze spectroscopic properties of Er3+ ions in glass hosts where various energy levels of Er3+ and transitions between them cannot be neglected.

Characterization of coherence properties of a free-running sub-kilohertz-linewidth fiber laser by FMCW

Frequency-modulated continuous wave (FMCW) measurements of Rayleigh backscattering in 85-km fiber derives coherence length of a free-running sub-kilohertz-linewidth fiber laser to be 210 km and the longest distance is measured with FMCW.

High power narrow linewidth fiber lasers

Compact, robust, single-frequency fiber lasers, emitting around 1.06 or 1.5 µm with up to several hundred mW power combine polarized output with very narrow linewidth of 1-3 kHz and fast frequency modulation up 10 kHz.