Improved λ/4 phase-shifted DFB semiconductor laser with spatial hole burning compensation using grating chirp

A λ/4 phase-shifted distributed feedback (DFB) semiconductor laser with a preformed chirped grating used to compensate the spatial hole burning (SHB) induced index change is proposed and analyzed. It shows that compared with the three phase shifted DFB laser which is known for its good performance to eliminate SHB, the proposed SHB compensated laser has better single longitudinal mode property, narrower spectral linewidth and better dynamic characteristics.     

A single-mode cw dye laser and the spatial hole burning effect

Single-mode operation of a cw dye laser apparently due to the spatial hole burning effect has been obtained by using a thin metallic absorbing film.     

Single-frequency ytterbium-doped fiber laser stabilized by spatial hole burning

We have exploited spatial hole burning to achieve remarkably stable single-frequency operation and mode-hop-free tuning over 300 free spectral ranges in an ytterbium-doped fiber laser with a sample standing-wave geometry. This approach makes possible stable and narrow-linewidth single-frequency fiber lasers that do not require components such as Faraday isolators, fiber couplers, and Fabry-Perot filters.     

Historical note on spatial hole burning and twisted-mode laser resonators

Optical information depends normally on one or two dimensions. A parallel transmission through a zero-dimentional fiber therefore needs a suitable coding. For an achromatric grey level object this can be achieved by the use of wavelength multiplexing. The information of each position in the original object is carried by a different wavelength. Simple dispersive elements perform the encoding and decoding process. Application examples are shown for the transmission of bar codes and paper tape data.     

Continuous-wave mode-locked solid-state lasers with enhanced spatial hole burning

We systematically investigate the difference between both actively and passively mode-locked lasers with Gain-at-the-End (GE) and Gain-in-the-Middle (GM) at the example of Nd:YLF lasers. The GE laser generates pulse widths approximately three times shorter than a comparable GM cavity. This is due to enhanced Spatial Hole Burning (SHB) which effectively flattens the saturated gain and allows for a larger lasing bandwidth compared to a GM cavity. We first investigate enhanced SHB by measuring the cw mode spectrum, where we have observed that the mode spacing in GE cavities depends primarily on the crystal length. This was also confirmed for a Nd:LSB crystal, where the pump absorption length was significantly shorter than the crystal length. In mode-locked operation, pulse widths of 4 ps for passive mode locking and 5 ps for active mode locking are demonstrated with GE cavities, compared to 11 ps for passive and 17 ps for active mode locking with GM cavities. Additionally, the time-bandwidth product for the GE cavity is approximately twice the ideal product for a sech² pulse shape and cannot be improved by dispersion compensation alone, while the GM cavity has nearly ideal time-bandwidth-limited performance. The results for the GM cavity compare well to existing theories taking into account the added effect of pump-power-dependent gain bandwidth which increases the bandwidth of Nd: YLF from 360 to > 500 GHz. In a following paper [1] (called Part II) a rigorous theoretical treatment of the effects due to SHB will be presented.     

Continuous-wave mode-locked solid-state lasers with enhanced spatial hole burning

In Part I of this paper [1] experimental results were presented and discussed. In this part, we investigate theoretically the dynamics of end-pumped solid-state lasers due to enhanced spatial hole burning. This becomes possible by a fast numerical implementation of the saturated gain in the presence of strong spatial hole burning that allows to treat the multimode case for an arbitrary pumping level. We find for a wide range of laser parameters that the mode spacing of the cw running modes is essentially determined by the length of the gain medium and only weakly depends on the absorption depth of the pump transition. It is shown that spatial hole burning can lead to a completely flat saturated gain profile over half of the gain bandwidth. In mode-locked lasers, the flat gain due to spatial hole burning results in shorter pulses. But the pulses are neither Gaussian-nor sech-shaped as they are in actively or passively mode-locked lasers without spatial hole burning. Further, we show that soliton-like pulse shaping can be used to restore a transform-limited sech-shaped pulse in an end-pumped solid-state laser while exploiting the full gain bandwidth of the laser material.     

High-pulse-energy passively mode-locked Nd:YVO4 laser without spatial hole burning effect

We report on a high-pulse-energy 888 nm diode-pumped passively mode-locked TEM₀₀ Nd:YVO₄ oscillator without spatial hole burning (SHB) effect in active medium. At a pump power of 40 W, the oscillator produced pulse energy up to 270 nJ at a repetition rate of 50 MHz, corresponding to average power of 13.5W with 34% optical-to-optical efficiency. To the best of our knowledge, it is the highest pulse energy obtained at the same pump power. The pulse duration produced varied from 16 to 59 ps. By varying the pump power, we observed a pulse duration ranging from 52 to 59 ps while the time-bandwidth product remained 0.88, and the factors that affect the pulse duration were also discussed. Meanwhile, a novel method of adjusting output coupling (OC) was introduced, which can simplify the structure of laser.     

The ridge waveguide fabrication with periodically poled MgO-doped lithium niobate for green laser

Ridge waveguides were fabricated using an external field, a precision lapping machine and neutron loop discharge (NLD) in magnesium-oxide-doped lithium niobate. The measured quasi-phase-matching (QPM) wavelength of the second-harmonic generation (SHG) in a 30 mm long periodically poled magnesium-doped lithium niobate (PPMgLN) ridge waveguide which has a domain period of 6.8 μm is about 532 nm. A fabricated periodically poled magnesium-doped lithium niobate ridge waveguide was duty cycle of 51.9 ± 2.83% and demonstrated second-harmonic generation. By using this periodically poled magnesium-doped lithium niobate ridge waveguide, highly effective, low-cost optical devices with high power or short wavelength can be achieved.     

New self-align method for fabrication of 980 nm ridge waveguide laser diodes

A novel self-align method has been developed for the fabrication of 980 nm ridge waveguide laser diodes. It utilizes the facts that (1) the thickness of photoresist on the ridge top is substantially less than that in its vicinity and (2) their respective exposure times differ accordingly. Except for replacing the second photolithographic step with a simple flood-exposure, the fabrication procedure is identical to that for conventional ridge waveguide laser diodes. No additional materials or processing steps are required. As a result, the laser fabrication is significantly simplified with excellent reproducibility.     

Infrared hole burning in matrix-isolated 1,2-difluoroethane with a tunable diode laser

Infrared hole burning within the absorption profile of the ν₁₇ vibrational transition of 1,2-difluoroethane matrix-isolated in solid Ar, Kr, and N₂ was observed. These measurements allowed the determination of the homogeneous and inhomogeneous linewidth ($\text{Δ}\text{ν}\text{?}{}_{\mathrm{<mtext>h</mtext>}}\text{= 0.002}\text{cm}{}^{\mathrm{?1}}$ at 2.5 K in Kr, $\text{Δ}\text{ν}\text{?}{}_{\mathrm{<mtext>i</mtext>}}\text{= 0.25}\text{cm}{}^{\mathrm{?1}}$). The temperature dependence of the homogeneous linewidth is explained in terms of vibrational relaxation as well as dephasing processes. A detailed analysis of the changes in the absorption profile with irradiation and calculation of the potential energy surface for rotation of the molecule in the matrix cage suggest a reorientation of the molecules in the matrix to be the cause of the observed hole burning.     

Thermal tuning and modulation of a DFB fibre laser with a thin-film heater

Results are given for thermal tuning and modulation of a 1556-nm distributed feedback fibre laser by resistive heating of a thin silver film chemically deposited on the fibre. Without reaching the limits of performance, linear tuning is demonstrated at a rate of 1.72 pm/mW up to about 200 pm, and a peak-to-peak modulation of 100 MHz up to modulation frequencies of 60 Hz. The heat flow is analyzed, and the coated fibre is characterized in terms of the static and dynamic wavelength response to the applied electric power. The performance of the scheme is tested by recording part of the ν₁+ν₃ combination band spectrum of ¹³C₂H₂ with thermal modulation and scanning of the fibre laser. Received: 12 March 2002 / Revised version: 24 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +45/4593-1137, E-mail: sus@dfm.dtu.dk     

A comparison of the intensity contours of fringes due to multiple reflection and spatial hole burning in semiclassical theory of laser

A comparison between spatial burning in the semiclassical theory of laser and the intensity contours of the fringes due to multiple reflections in a Fabry-Perot cavity is presented. The concept of spatial hole burning is also used in a quantum well system.     

Dynamics of a waveguide pulsed-periodic CO2 laser with loss modulation

The nonlinear dynamics of a waveguide CO₂ laser with combined periodic loss and pumping modulation is investigated theoretically and experimentally. The possibilities of controlling time and energy parameters of the output radiation using the corresponding selection of the detuning of the cavity's longitudinal mode from the center of the amplification line, the amplitude, and the frequency of loss modulation are demonstrated. The observed asymmetric dependence of the lasing-radiation density on the detuning of the cavity frequency is explained by the attendant modulation of the refractive index of the loss modulator. To whom correspondence should be addressed. B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 471–478, July–August, 1999.     

A double wavelength DFB laser with an identical active area for CWDM

A novel distribute feedback (DFB) laser which gave two different wavelengths under two distinct work conditions was fabricated. The laser consists of two Bragg gratings with different periods corresponding to wavelength spacing of 20 nm in an identical active area. When driving current was injected into one of the different sections separately, two different wavelengths at 1542.4 and 1562.5 nm were realized. The side mode suppression ratio (SMSR) of 45 dB or more both for the two Bragg wavelengths were achieved. The fabricating process of the laser was just the samp as that of traditional DFB laser diode. This device can be potentially used in coarse wavelength division multiplexer (CWDM) as a promising light source and the technology idea can be used to enlarge the transmission capacity in metro area network (MAN).     

A double wavelength DFB laser with an identical active area for CWDM

A novel distribute feedback (DFB) laser which gave two different wavelengths under two distinct work conditions was fabricated. The laser consists of two Bragg gratings with different periods corresponding to wavelength spacing of 20 nm in an identical active area. When driving current was injected into one of the different sections separately, two different wavelengths at 1542.4 and 1562.5 nm were realized. The side mode suppression ratio (SMSR) of 45 dB or more both for the two Bragg wavelengths were achieved. The fabricating process of the laser was just the same as that of traditional DFB laser diode. This device can be potentially used in coarse wavelength division multiplexer (CWDM) as a promising light source and the technology idea can be used to enlarge the transmission capacity in metro area network (MAN).     

Programmable frequency reference for subkilohertz laser stabilization by use of persistent spectral hole burning

We report what is believed to be the first demonstration of laser frequency stabilization directly to persistent spectral holes in a solid-state material. The frequency reference material was deuterated CaF(2): Tm(3+) prepared with 25-MHz-wide persistent spectral holes on the H(6)(3)?H(4)(3) transition at 798 nm. The beat frequency between two lasers that were independently locked to persistent spectral holes in separate crystal samples showed typical root Allan variances of 780+/-120Hz for 20-50-ms integration times.     

Holographic DFB laser with fan-shaped grating for a spectrofluorimeter

A method for recording fan-shaped transmitting phase holographic gratings that ensure tuning of the wavelength of emission for a DFB (distributed-feedback) dye laser is presented. Traditional optical elements are used in the scheme of grating recording. A change in the degree of grating fanning is reached here without replacement of gratings. The possibility of obtaining frequency-tuned emission by means of the fan-shaped gratings created is demonstrated experimentally. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 48–51, January–February, 2000.     

采用QPSK调制的50 Gbit/s高速大气激光通信传输特性研究

高级多电平调制格式在海上高速和高光谱效率的光通信中已经显示出巨大的潜力。将正交相移键控(QPSK)调制格式引入自由空间光(FSO)通信,实验证明可实现高达50 Gbit/s的高速FSO传输链路,能够清楚地观察到所发送的QPSK信号的眼图和星座图。通过比较传输之前和之后的误码率曲线,可以发现接收灵敏度都小于-36.7 dBm。当误码率为3.8×10-3时,它们的功率损失都小于4.2 dB。     

96 mW longitudinal single mode red-emitting distributed Bragg reflector ridge waveguide laser with tenth order surface gratings

Red-emitting ridge waveguide lasers with integrated tenth order surface distributed Bragg reflector gratings were developed. The grating was implemented by the use of a BCl3-Ar-plasma, while the shape of the grating trench was controlled by additional He-backside cooling of the wafer. The devices exhibit longitudinal single mode operation up to 96 mW at 635.3 nm with a side mode suppression ratio of 18 dB and a good beam quality of M2<3. The spectrum is free of mode hops for a span of more than 55 pm.     

A fibre grating DFB laser for generation of optical microwave signal

A distributed feedback fibre laser is realised with a phase-shifted fibre Bragg grating. Dual-mode operation is obtained by means of linear birefringence for heterodyne generation of optical microwave signal. The two modes, operating in the same cavity produce a beat signal with exceptionally low-phase noise. The linewidth of the resulting microwave signal obtained is less than 1 kHz.