Characteristics of a saturated 18.9-nm tabletop laser operating at 5-Hz repetition rate

We report the characteristics of a saturated high-repetition rate Ni-like Mo laser at 18.9 nm. This table-top soft X-ray laser was pumped at a 5-Hz repetition rate by 8-ps 1-J optical laser pulses impinging at grazing incidence into a precreated Mo plasma. The variation of the laser output intensity as a function of the grazing incidence angle of the main pump beam is reported. The maximum laser output intensity was observed for an angle of 20/spl deg/, at which we measured a small signal gain of 65 cm/sup -1/ and a gain-length product g/spl times/l>15. Spatial coherence measurements resulting from a Young's double-slit interference experiment show the equivalent incoherent source diameter is about 11 /spl mu/m. The peak spectral brightness is estimated to be of the order of 1/spl times/10/sup 24/ photons s/sup -1/ mm/sup -2/ mrad/sup -2/ within 0.01% spectral bandwidth. This type of practical, small scale, high-repetition soft X-ray laser is of interest for many applications.     

Investigating up-conversion fluorescence of Phloxine B

The steady-state and time-resolved emission of Phloxine B was examined when excited with 90 fs pulses from a mode-locked Ti:sapphire laser. Above the excitation wavelength (775 nm), Phloxine B was found to display two-photon excitation fluorescence with estimated cross-section for excitation of 0.87×10^-49 cm⁴ s/photon, and fluorescence spectrum consistent with one-photon excitation. Over the excitation wavelength range from 590 to 650 nm, Phloxine B was found to display one-photon excitation up-conversion fluorescence. The up-conversion fluorescence of Phloxine B was confirmed by frequency-domain measurements. The intensity decays revealed different double-exponential intensity decays for one-photon excitation at 390 nm and 633 nm. The longer fluorescence lifetimes were observed with 633 nm excitation. Hence, anti-Stokes excitation of Phloxine B causes delay emission at shorter wavelengths. Time-resolved anisotropy decay measurements revealed similar correlation times, but different amplitudes, as has been observed previously for two- versus one-photon excitation     

High-power continuous-wave intracavity frequency-doubled Nd:GdVO/sub 4/-LBO laser under diode pumping into the emitting level

The continuous-wave high power laser emission of Nd:GdVO/sub 4/ at the fundamental wavelength of 1.06 /spl mu/m and its 531-nm second harmonic obtained by intracavity frequency doubling with an LBO nonlinear crystal is investigated under pumping by diode laser at 808 nm (on the /sup 4/I/sub 9/2//spl rarr//sup 4/F/sub 5/2/ transition) and 879 nm (on the /sup 4/I/sub 9/2//spl rarr//sup 4/F/sub 3/2/ transition). It is shown that, in spite of a lower absorption at 879 nm, the infrared emission is comparable under these two wavelengths of pump. The green emission performances were, however, improved by the 879 nm pump: 5.1 W at 531 nm with M/sup 2/=1.46 and 0.31 overall optical-to-optical efficiency was obtained from a 3-mm-thick 1-at.% Nd:GdVO/sub 4/ laser medium and a 10-mm-long LBO nonlinear crystal in a Z-type cavity for 16.5 W pump power. In similar conditions, the maximum green power for the 808 nm pump was 4.4 W, with 0.26 overall optical-to-optical efficiency and M/sup 2/=3.40 beam quality; at this pump wavelength the green emission shows evident saturation for pump power in excess of 9.9 W. This behavior is connected with the enhanced heat generation under 809-nm pumping, as evidenced by the increased thermal lensing of the fundamental emission. A careful alignment of the laser enables emission almost free of chaotic intensity fluctuations.     

Ultrafast semiconductor-based fiber laser sources

In this paper, a novel ring laser platform is presented that uses a single active element, a semiconductor optical amplifier (SOA), to provide both gain and gain modulation in the optical cavity. Gain modulation is achieved by an externally introduced optical pulsed signal. This signal periodically saturates the amplifier gain and forces the ring laser to mode lock. Using this laser platform, we demonstrate picosecond pulsetrain generation at repetition rates up to 40 GHz, either in single or multiwavelength operation mode. In particular, using rational harmonic mode locking, 2.5-ps pulses were obtained up to a 40-GHz repetition rate, while output pulses and output power were constant over a 20-nm tuning range. In addition, a multiwavelength optical signal was obtained using the same laser platform with the addition of a Fabry-Pe/spl acute/rot filter for comb generation. Multiwavelength oscillation is possible due to the broad gain spectrum of the SOA used and its inhomogeneous line broadening. To this end, 48 oscillating wavelengths were obtained at the laser output, with 50-GHz line spacing. Combining both modes of operation, it was possible to mode lock the oscillating multiwavelength signal and to obtain at the output ten wavelength channels, simultaneously mode locked at a 30-GHz repetition rate. The mode-locked channels are temporarily synchronized and exhibit almost identical spectral and time characteristics.     

Calibration of a calorimeter for measuring the spark energy of an electrostatic discharge

This paper deals with the factors that influence the accuracy of the measurements performed with a calorimeter, developed to detect the optical signal emitted by the spark generated to facilitate experiments with electrostatic discharge. Experiments are described which were performed with a special calorimeter and a compound optical source consisting of a spark gap, a human-body-model electrostatic discharge (ESD) signal generator, and four LEDs. The spark gap was used as a versatile and more powerful source of optical emission than the human-body-model circuit; the LED source was used for calibration and alignment. For control, the spectrum of the spark discharge was measured with a spectrometer and a broadband photodetector. The calorimeter was used as a means for the determination of the energy contained in the optical signal, yielding the value of the spark-gap emission in the fundamental units for energy. The long-term stability of the system was measured, and the system response was studied for threshold optical signals. The calorimeter detectivity, defined as the overall instrument sensitivity, measured in volts per joule, was D=4.3/spl times/10/sup 7/ V.J/sup -1/ and its detection limit 2.3/spl times/10/sup -13/ J. The system's response to the IEC standard human-body-model circuit was consistent with the measurements of the system's detectivity. Most parts of the developed apparatus have been tested in an industrial environment.     

GaInNAsSb for 1.3-1.6-/spl mu/m-long wavelength lasers grown by molecular beam epitaxy

High-efficiency optical emission past 1.3 /spl mu/m of GaInNAs on GaAs, with an ultimate goal of a high-power 1.55-/spl mu/m vertical-cavity surface-emitting laser (VCSEL), has proven to be elusive. While GaInNAs could theoretically be grown lattice-matched to GaAs with a very small bandgap, wavelengths are actually limited by the N solubility limit and the high In strain limit. By adding Sb to the GaInNAs quaternary, we have observed a remarkable shift toward longer luminescent wavelengths while maintaining high intensity. The increase in strain of these new alloys necessitates the use of tensile strain compensating GaNAs barriers around quantum-well (QW) structures. With the incorporation of Sb and using In concentrations as high as 40%, high-intensity photoluminescence (PL) was observed as long as 1.6 /spl mu/m. PL at 1.5 /spl mu/m was measured with peak intensity over 50% of the best 1.3 /spl mu/m GaInNAs samples grown. Three QW GaIn-NAsSb in-plane lasers were fabricated with room-temperature pulsed operation out to 1.49 /spl mu/m.     

InP-GaInP quantum-dot lasers emitting between 690-750 nm

We describe the growth, material characterization, and device characterization of InP-GaInP quantum-dot lasers for operation in the wavelength range 690-750 nm. We show that the growth conditions have a major influence on the form of the gain spectrum. Relatively flat gain can be achieved over a spectral width of 90 nm at 300 K using samples containing a bimodal distribution of dot sizes, or narrower gain spectra at shorter wavelength can be achieved by suppressing the bimodal distribution by using (211)B substrates. Optimization of samples grown on substrates with the growth surface of (100) misorientated by 10/spl deg/ toward [111] results in laser operation between 729 and 741 nm and with a room temperature threshold current density as low as 190 A/spl middot/cm/sup -2/ for a 2000-/spl mu/m-long device with uncoated facets.     

Acoustic response of cartilage during laser-mediated stress relaxation

In this study, laser reshaping of porcine septal cartilage was performed using an Nd:YAG laser (/spl lambda/=1.32 /spl mu/m), while changes in acoustic waves were observed, in an attempt to develop technique to monitor the reshaping process. Concurrent measurements of strain (during tensile compression and tension, as well as flexure), temperature, and a 5-MHz ultrasonic signal were recorded during laser irradiation (/spl lambda/=1.32 /spl mu/m, 4 W, 13 s). The sample was set up in a water bath to enhance acoustic coupling. From the ultrasonic signal, both time of flight (TOF) and signal amplitude as reflected from the back wall of the cartilage were extracted and correlated with temperature and strain measurements. The onset of stress relaxation of the cartilage generally occurred between 50/spl deg/C and 60/spl deg/C. While TOF measurements indicated a generally constant increase in the speed of sound of the cartilage during the irradiation period, the amplitude of the reflected acoustic signal correlated directly with the stress relaxation of the cartilage. At the point of stress relaxation, the amplitude of the acoustic signal consistently attenuated to roughly 50% of its original magnitude.     

Analysis of noise mechanisms limiting the frequency stability of microwave signals generated with a femtosecond laser

Excess phase noise is observed in the spectrum of the microwave signal extracted from a photodetector illuminated by a train of ultrashort light pulses from the femtosecond laser. This noise affects the stability of frequency transfer from optical to microwave domains with the femtosecond laser. Some contributions to the excess phase noise are related to intrinsic beam-pointing fluctuations of the femtosecond laser and optical power fluctuations of the detected light. These factors contribute to excess phase noise at the harmonics of the pulse repetition rate due to power-to-phase conversion in the photodetector, spatially dependent time delays, and photodiode nonlinearities that distort the pulse shape. With spatial filtering of the laser beam and active control of its power, the additional fractional frequency fluctuations of pulse repetition rate associated with the excess noise of the photodetection process were reduced from 6/spl middot/10/sup -14/ to approximately 3/spl middot/10/sup -15/ over 1 s of averaging. The effects of other noise mechanisms, such as laser shot noise and phase noise introduced by a microwave amplifier, were also examined but were found to be at a less significant level.     

A new method to determine the absolute mode number of a mode-locked femtosecond-laser comb used for absolute optical frequency measurements

The generation of optical frequency combs, directly referenced to the SI second, can be used to make measurements of optical frequencies. This provides a supreme method for the realization of the meter. However, an approximate knowledge of the frequency of the radiation is normally needed for such measurements in order to determine the integral order of the comb component used. Such information is usually obtained by prior wavelength measurements of the radiation under study. This paper demonstrates a new method to determine the absolute mode number in optical frequency measurements using mode-locked femtosecond lasers, thus eliminating the need for complementary wavelength measurements. Measurements of the frequency of an iodine-stabilized He-Ne laser at /spl lambda/=633 nm and a Nd:YAG laser at /spl lambda/=532nm are given as examples.     

Performance of 20 Gb/s quaternary intensity modulation based on binary or duobinary modulation in two quadratures with unequal amplitudes

2/spl times/10 Gb/s quaternary intensity modulation signals (4-IM) can be generated by combining two modulation signals with unequal amplitudes in quadrature phases or orthogonal polarizations. Two 10-Gb/s nonreturn-to-zero (NRZ) amplitude-shift keying (ASK) signals and a quadrature phase-shift keying (QPSK) modulator allow to generate 4-IM with the same bandwidth as an NRZ-ASK signal (QASK). Measured sensitivity at a bit error rate (BER) of 10/sup -9/ and chromatic dispersion (CD) tolerance are -21.6 dBm and /spl sim/+130 ps/nm, respectively. Two duobinary 10-Gb/s data streams and a QPSK modulator allow to generate a 9-constellation point quaternary intensity signal (QDB), with the same bandwidth as a duobinary signal. A stub filter with frequency response dip at 5 GHz was used to generate the duobinary signals. Detected as a 4-IM, this scheme features a sensitivity and a CD tolerance of -21.2 dBm and /spl sim/+140 ps/nm, respectively. By combining the two duobinary 10-Gb/s data streams with unequal amplitudes in orthogonal polarizations, a 9-constellation point quaternary intensity signal was also obtained (QPolDB). Sensitivity and CD tolerance were -20.5 dBm and /spl sim/+340 ps/nm, respectively. They became -18.4 dBm and /spl sim/+530 ps/nm, respectively, when the frequency response dip of the stub filter was changed to 6 GHz. A polarization and phase-insensitive direct detection receiver with a single photodiode has been used to detect all generated quaternary signals as 4-IM signals.     

Optical signal inversion phenomena due to negative nonlinear absorption effect

A negative input-output characteristic was obtained in an erbium-doped yttrium aluminum garnet crystal which has multiple-level energy structure: absorption can occur between a ground level and an excited level (⁴I−⁴I) and between two other excited levels (⁴I−²H) in Er³⁺. It occurred at a wavelength between 786.6 and 788.4 nm by variation of the incident laser intensity in the intensity range of 60 nW/cm² to 100 W/cm². It is considered that an enhanced absorption occurs due to the excited-state absorption from ⁴I to ²H, its spectra matching the wavelength dependence of the incident laser In addition, optical signal inverter phenomenon was observed using a laser diode modulated at 10 MHz. By considering multiple excited-state absorption in detail, active functions such as an optical signal inverter could be derived from the negative nonlinear absorption effect. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(4): 74–80, 1997     

High-average power EUV light source for the next-generation lithography by laser-produced plasma

Laser-produced plasma is expected to fulfill the strict requirement as an extreme ultraviolet (EUV) light source for the next-generation lithography with 115-W average power at the intermediate focus, in terms of stability, minimum contamination, and cost of ownership. A liquid xenon micro jet is employed in our experimental facility to confirm the scalability up to the 115-W clean output power. The present experimental device is composed of a 1-kW 10-kHz 6-ns Nd:YAG laser with a xenon jet of up to 50-/spl mu/m diameter of 35 m/s speed inside a vacuum chamber, combined with a xenon recirculation system. The observed EUV power is 4 W at 13.5 nm (2% bandwidth, 2/spl pi/sr) from the plasma source with 0.5% stability (1 /spl sigma/, 50-pulse moving average). Debris mitigation and contamination control is now studied for fast ions by time-of-flight measurements, and confinement and exhaust by a magnetic field is shown to be effective. Xe/sup +/ to Xe/sup 13+/ ions were observed with Xe/sup 2+/ being the main charged state. Experimental study is presented on these subjects and further parametric study on the laser wavelength and pulsewidth is reported, discussing the scalability toward the realization of a 115-W system.     

Universal properties of random lasers

The design and fabrication of laser resonators is often difficult. However, random lasers occur in gain media with numerous scatterers and produce coherent laser emission without any predesigned cavity. The generation of coherent emission from multiple scattering is quite general and its basic principles are shown here using two model systems, namely /spl pi/-conjugated polymer films and rhodamine-TiO/sub 2/ suspensions. Above a threshold excitation intensity, both systems show narrow emission lines (<0.5 nm), coherence that is determined by photon statistics, and a fundamental cavity length in the disordered material that is revealed by averaging multiple power Fourier transform spectra.     

差频产生中红外光源及甲烷气体光谱检测

连续波可调谐中红外光源在大气探测、工业生产、医学诊断等领域具有重要的应用价值。基于准相位匹配(QPM)差频产生(DFG)技术,以1060和1550nm波段窄线宽激光器作为基频光源,在周期极化铌酸锂(PPLN)晶体中获得了中红外激光输出。实验系统研究了该中红外DFG光源的转化效率、温度和波长调谐特性。在此基础上,基于所设计的中红外DFG光源构建了甲烷气体检测系统,通过扫描泵浦光波长获得了甲烷分子v3基频振动带2 999.01cm~(-1)处的吸收光谱。     

Investigation of near-infrared autofluorescence imaging for the detection of breast cancer

Detection of breast cancer in fresh tissue obtained from surgery is investigated using near-infrared autofluorescence imaging under laser excitation at 532 and 632.8 nm. The differences in intensity between the three main components of breast tissue (cancer, fibrous, and adipose) are estimated and compared to those obtained from cross-polarized light scattering images recorded under polarized illumination at 700 nm. The optical spectroscopic images for each tissue sample were subsequently compared with the histopathology slides. The experimental results indicate that the intensity of the near-infrared emission is considerably different in breast cancer compared to that of the adjacent nonneoplastic tissues (adipose and fibrous tissue). The experimental results suggest that 632.8-nm excitation offers key advantages compared to 532 nm excitation.     

Tuning range extension by active mode-locking of external cavity laser including a linearly chirped fiber Bragg grating

We demonstrate electrical wavelength tuning by mode locking of an external cavity laser (ECL) with linearly chirped fiber Bragg grating (LCFBG). The configuration consists of a laser chip providing the gain coupled to an LCFBG with a large chip rate of 10 or 55 nm/cm providing the counter-reaction for laser oscillation. The laser chirp is electrically modulated by a sinusoidal signal in such a way that the ECL is mode locked. By changing the modulation frequency, a wavelength tuning range of 27 nm is achieved with the 10 nm/cm LCFBG, and a partial tuning range over 41 nm is demonstrated with the 55-nm/cm LCFBG. The output pulse stream at a specific mode-locking frequency and a corresponding wavelength is obtained for both positively and negatively chirped grating. A time bandwidth product reduction is measured in the case of negatively chirp grating when compared with positively chirp grating. A simple general law between the laser parameters is given (locking frequency, tuning range, and FBG chirp value). The parameters for a 40-nm tunable source modulated at 10 GHz are given. This simple tuning mechanism is very well adapted for a structure that requires accurate wavelength monitoring.     

Single-mode distributed feedback and microlasers based on quantum-dot gain material

Quantum-dot gain material fabricated by self-organized epitaxial growth on GaAs substrates is used for the realization of 980-nm and 1.3-/spl mu/m single-mode distributed feedback (DFB) lasers and edge-emitting microlasers. Quantum-dot specific properties such as low-threshold current, broad gain spectrum, and low-temperature sensitivity could be demonstrated on ridge waveguide and DFB lasers in comparison to quantum-well-based devices. 980-nm DFB lasers exhibit stable single-mode behavior from 20/spl deg/C up to 214/spl deg/C with threshold currents < 15 mA (1-mm cavity length). Utilizing the low-bandgap absorption of quantum-dot material miniaturized monolithically integrable edge-emitting lasers could be realized by deeply etched Bragg mirrors with cavity lengths down to 12 /spl mu/m. A minimum threshold current of 1.2 mA and a continuous-wave (CW) output power of >1 mW was obtained for 30-/spl mu/m cavity length. Low-threshold currents of 4.4 mA could be obtained for 1.3-/spl mu/m emitting 400-/spl mu/m-long high-reflection coated ridge waveguide lasers. DFB lasers made from this material by laterally complex coupled feedback gratings show stable CW single-mode emission up to 80/spl deg/C with sidemode suppression ratios exceeding 40 dB.     

Effect of thermal damage on the in vitro optical and fluorescence characteristics of liver tissues

Thermal energy generated by radio-frequency current or other means may be employed in treating liver tumors by means of thermal coagulation when conventional resection is impossible. Currently, these thermal energy-based therapeutic procedures suffer from the lack of an adequate feedback control system, making it difficult to determine the optimal therapeutic endpoint. In this study, the potential of optical spectroscopy to provide such an objective endpoint for these procedures is presented. Freshly harvested canine liver samples were exposed to 50/spl deg/C, 60/spl deg/C, and 70/spl deg/C water baths for times ranging from 0 to 60 min. Transmission and reflectance were measured from each sample using an integrating sphere and the optical properties of each sample were accordingly derived. Excitation-emission matrices were recorded from the samples using a spectrofluorometer to identify the intrinsic fluorescence characteristics of native and thermally coagulated liver tissues. In addition, fluorescence and diffuse reflectance spectra were separately obtained from the samples prepared using a portable spectroscopic system. Results of this study show that fluorescence and optical properties of liver tissues exhibit clear and consistent changes through the thermal coagulation process. Specifically, the primary peak in the fluorescence spectra from liver tissues shifts from 480 nm in the native state to 510 nm in the fully coagulated state. In addition, a three- to fourfold increase in the absolute intensity of the diffuse reflectance spectra is observed upon complete coagulation of liver tissues. These dynamic spectral features indicate that fluorescence and diffuse reflectance spectroscopy may provide a direct measure of the biochemical and structural changes associated with tissue thermal damage in the liver.     

Growth of highly strained GaInAs-GaAs quantum wells on patterned substrate and its application for multiple-wavelength vertical-cavity surface-emitting laser array

We carried out the growth of highly strained GaInAs-GaAs quantum wells (QWs) on a patterned substrate for extending emission wavelength on a GaAs substrate. We examined the shift of photoluminescence wavelength of the QWs and showed a large wavelength shift due to the spatial modulation in well thickness and indium composition. We demonstrated a single-mode multiple-wavelength vertical-cavity surface-emitting laser (VCSEL) array on a patterned GaAs substrate covering a new wavelength window of 1.1-1.2 /spl mu/m. By optimizing pattern shape, we achieved multiple-wavelength operation with widely and precisely controlled lasing wavelengths. The maximum lasing span is as large as 77 nm. We carried out a data transmission experiment through 5-km of single-mode fiber with a 2.5 Gb/s/channel. The total throughput reaches 10 Gb/s. The VCSEL-based wavelength-division-multiplexing (WDM) source would be a good candidate for WDM-LAN beyond 10 Gb/s.