Laurence Watkins

High-speed noncontact fiber-diameter measurement using forward light scattering

The forward scattering of light by an optical fiber produces an interference fringe pattern, and the fringe period is inversely proportional to the fiber diameter. An electrooptic system has been developed to produce and detect this scattering pattern to provide an instrument which will measure fiber diameter during the drawing operation. The system measures the fiber diameter at a 1-kHz rate with a precision of 0.25 microm and an accuracy of +/-0.25 microm over a range of 50-150-microm diams. The instrument allows the fiber to move laterally in a 1-cm diam window maintaining the above accuracy. The system can be calibrated optically and does not need a standard fiber for this procedure. The instrument has been used for months without the need for recalibration. In addition to the digital diameter output, the system employs a microprocessor to compute mean and standard deviation values for various sample lengths and provides suitable signals for feedback control of fiber diameter.

Laser beam refraction traversely through a graded-index preform to determine refractive index ratio and gradient profile

A technique is described which permits the determination of geometric and refractive index characteristics of graded-index preforms from measurements of the refraction of rays traced through the preform perpendicular to the preform axis. A computer program was developed to trace rays through graded-index preforms to display the refracting effect of the index properties and relate the ray incidence angle to its deflection in traversing the preform. An experimental apparatus has been developed in which a narrow beam of laser radiation is directed at the preform and its deflection angle measured. Comparison between the experimental results and the ray trace calculations using an interative curve-fitting procedure gave nondestructive determinations of the refractive index ratio Delta and index gradient profile parameter alpha as well as measurement of the core dimensions.

Fiber geometry specifications and its relation to measured fiber statistics

Fiber quality requirements can be specified in a number of ways; the most typical is to state a required diameter and give a tolerance band to allow for manufacturing variations. Actual fiber diameter statistics have been shown to be close to Gaussian for a certain range of fiber lengths. These fiber statistics are discussed showing the bounds of stationarity and their relation to fiber specifications. Thus the fiber statistics in terms of standard deviation and sample length can be related to the specification. Not only variations within one fiber are important, but also from fiber to fiber. Thus, the effect of mean-to-mean differences in fibers and their influence in meeting fiber specification for given standard deviations are considered. In considering these factors, the following conclusions are made. There is a bandwidth which can be related to the fiber drawing process, and measurement of fiber diameter for the purpose of characterization must be made at a rate greater than this bandwidth to guarantee fiber quality. Since fiber drawing is a predominately random process, specifications should be written which relate to a statistical description of the process. If a tolerance band is used for the specification of fiber diameter, it ought to comprise two parts. One part will allow for the random fiber diameter variations. The second part must provide for the deviations of the mean diameter of each complete length from the nominal.

High power 808 nm VCSEL arrays for pumping of compact pulsed high energy Nd:YAG lasers operating at 946 nm and 1064 nm for blue and UV light generation

High power 808 nm VCSEL arrays were developed to pump compact pulsed Nd:YAG lasers. A QCW side-pumped passively Q-switched Nd:YAG laser operating at 1064 nm produced linearly polarized 4 ns IR pulses with 4.7 mJ pulse energy. These pulses were externally frequency doubled and quadrupled resulting in 2.5 mJ pulse energy at 532 nm and 0.8 mJ at 266 nm respectively. A similar but actively Q-switched dual side-pumped Nd:YAG laser operating at the weaker quasi three-level 946 nm transition produced 12 mJ pulses that were efficiently frequency doubled resulting in 5.6 mJ blue pulses of 17 ns duration.

Low noise high power ultra-stable diode pumped Er-Yb phosphate glass laser

We are developing a low noise high power ultra-stable diode pumped Er-Yb co-doped phosphate glass laser. Erbium doped phosphate glass permits high co-doping with ytterbium ions that strongly absorb at 976 nm and efficiently transfer their energy to the active erbium material. This drastically decreases the absorption length at the 976 nm pump wavelength and thus the overall size of the laser. Aside from the advantage for packaging a short cavity length results in a large longitudinal mode-spacing (>40 GHz), which allows for single longitudinal mode operation in the 1530-1565 nm C-band for telecommunication by inserting a tunable low-finesse etalon in the laser cavity. In addition, due to the energy transfer between the co-dopant and the active material, the laser shows a strongly reduced sensitivity to fluctuations in pump power. The strong peak in the RIN spectrum at the relaxation oscillation frequency (0.1-1 MHz) due to cavity-loss perturbations can be drastically reduced with a non-linear absorbing material inside the laser cavity. Using this approach for an optimized laser cavity design we have achieved -160 dB/Hz RIN at 1 MHz for 35 mW output. Above 100 MHz the RIN becomes shot noise limited (-168 dB/Hz @ 20mA photocurrent). The laser has excellent long-term frequency stability when locked to our wavelength locker (<250 kHz). Furthermore, the laser has been shown to have a narrow intrinsic linewidth (~10 Hz) that we are working towards by means of intra-cavity phase modulation.

An Optical Fiber Diameter Measurement System Using Forward Scattered Light

Forward scattering of light by an optical fiber produces an interference fringe pattern in which the fringe spacing is inversely proportional to the fiber diameter. An electrooptic system has been developed to produce and detect this scattering pattern, thus providing a measure of fiber diameter.

Compact 4.7 W, 18.3% wall-plug efficiency green laser based on an electrically pumped VECSEL using intracavity frequency doubling.

We have demonstrated a compact, 4.7 W green laser based on an electrically pumped vertical external-cavity surface emitting laser through intracavity frequency doubling. The overall wall-plug efficiency (electrical to green) was 18.3%. The power fluctuations were measured to be ±1.4% over a 2 h time period.

High power VCSEL array pumped Q-switched Nd:YAG lasers

Solid-state lasers pumped by high-power two-dimensional arrays of vertical-cavity surface-emitting lasers (VCSELs) were investigated. Both end-pumping and side-pumping schemes of Nd:YAG lasers with high power kW-class 808 nm VCSEL pump modules were implemented. For one application 10 mJ blue laser pulses were obtained from a frequencydoubled actively Q-switched VCSEL-array dual side-pumped Nd:YAG laser operating at 946 nm. For another application 10 mJ green laser pulses were obtained from a frequency-doubled passively Q-switched VCSEL-array endpumped Nd:YAG laser operating at 1064 nm. Both QCW and CW pumping schemes were investigated to achieve high average Q-switched power.

Effect of noncircular cross section on the forwardscattering pattern of side-illuminated unclad fibers

The geometric ray-trace theory has been modified to predict the intensity maxima and minima of the forwardscattering pattern for unclad fibers with noncircular cross sections. The cross section is represented by a Fourier series, and the effect of the presence of various foils (harmonics) is demonstrated. The results show that the magnitude of low-order foils, specifically the second, third, fourth, fifth, and seventh, can be determined by observing the pattern at ~12 degrees scattering angle and monitoring the interference fringe movement as the fiber is rotated axially. The effects of elliptical fibers on fiber diameter determinations from measurement of the scattering pattern are discussed.

High power high repetition rate VCSEL array side-pumped pulsed blue laser

High power, kW-class, 808 nm pump modules based on the vertical-cavity surface-emitting laser (VCSEL) technology were developed for side-pumping of solid-state lasers. Two 1.2 kW VCSEL pump modules were implemented in a dual side-pumped Q-switched Nd:YAG laser operating at 946 nm. The laser output was frequency doubled in a BBO crystal to produce pulsed blue light. With 125 μs pump pulses at a 300 Hz repetition rate 6.1 W QCW 946 nm laser power was produced. The laser power was limited by thermal lensing in the Nd:YAG rod.