Juan Montoya

Nanometer-level repeatable metrology using the Nanoruler

We report on the measurement of the fringe-to-substrate phase error in our Nanoruler system. This system utilizes scanning beam interference lithography to pattern and measure large-area, nanometer-accuracy gratings that are appropriate for semiconductor and integrated opto-electronic metrology. We present the Nanonruler’s metrology system that is based on digital frequency synthesizers, acousto-optics, and heterodyne phase sensing. It is used to assess the fringe-to-substrate placement stability and the accuracy of the feedback signals. The metrology system can perform measurements in real time, on the fly, and at arbitrary locations on the substrate. Experimental measurements are presented that demonstrate the nanometer-level repeatability of the system. Dominant error sources are highlighted.

Surface plasmon isolator based on nonreciprocal coupling

Integrated photonics require optical isolators that achieve low insertion loss and large optical isolation. Here we describe a surface plasmon enhanced optical isolator based on nonreciprocal coupling from a dielectric waveguide coupled to a surface plasmon waveguide. The surface plasmon core consists of a magnetic metal which results in a large nonreciprocity, allowing for device lengths on the order of 50 μm. The analysis and modeling presented here indicate that greater than 30 dB isolation and less than 3 dB insertion loss are possible.

High fidelity blazed grating replication using nanoimprint lithography

We report progress in using nanoimprint lithography to fabricate high fidelity blazed diffraction gratings. Anisotropically etched silicon gratings with 200nm period and 7.5° blaze angle were successfully replicated onto 100mm diameter wafers with subnanometer roughness and excellent profile conformity. Out-of-plane distortion induced by residual stress from polymer films was also analyzed and found to be extremely low. The replicated blazed gratings were tested and demonstrated high x-ray diffraction efficiencies. This process was developed for fabricating blazed diffraction gratings for the NASA Constellation-X x-ray telescope.

Spectral behavior of a terahertz quantum-cascade laser

In this paper, the spectral behavior of two terahertz (THz) quantum cascade lasers (QCLs) operating both pulsed and cw is characterized using a heterodyne technique. Both lasers emitting around 2.5 THz are combined onto a whisker contact Schottky diode mixer mounted in a corner cube reflector. The resulting difference frequency beatnote is recorded in both the time and frequency domain. From the frequency domain data, we measure the effective laser linewidth and the tuning rates as a function of both temperature and injection current and show that the current tuning behavior cannot be explained by temperature tuning mechanisms alone. From the time domain data, we characterize the intrapulse frequency tuning behavior, which limits the effective linewidth to approximately 5 MHz.

External cavity beam combining of 21 semiconductor lasers using SPGD

Active coherent beam combining of laser oscillators is an attractive way to achieve high output power in a diffraction limited beam. Here we describe an active beam combining system used to coherently combine 21 semiconductor laser elements with an 81% beam combining efficiency in an external cavity configuration compared with an upper limit of 90% efficiency in the particular configuration of the experiment. Our beam combining system utilizes a stochastic parallel gradient descent (SPGD) algorithm for active phase control. This work demonstrates that active beam combining is not subject to the scaling limits imposed on passive-phasing systems.

High speed, high power one-dimensional beam steering from a 6-element optical phased array

Beam steering at high speed and high power is demonstrated from a 6-element optical phased array using coherent beam combining (CBC) techniques. The steering speed, defined as the inverse of the time to required to sweep the beam across the steering range, is 40 MHz and the total power is 396 mW. The measured central lobe FWHM width is 565 μrad. High on-axis intensity is maintained periodically by phase-locking the array via a stochastic-parallel-gradient-descent (SPGD) algorithm. A master-oscillator-power-amplifier (MOPA) configuration is used where the amplifier array elements are semiconductor slab-coupled-optical-waveguide-amplifiers (SCOWAs). The beam steering is achieved by LiNbO(3) phase modulators; the phase-locking occurs by current adjustment of the SCOWAs. The system can be readily scaled to GHz steering speed and multiwatt-class output.

Two-color-absorption sensor for time-resolved measurements of gasoline concentration and temperature

A midinfrared absorption sensor for crank-angle-resolved in-cylinder measurements of gasoline concentration and gas temperature for spark-ignition internal-combustion engines is reported, and design considerations and validation testing in the controlled environments of a heated cell and shock-heated gases are discussed. Mid-IR laser light was tuned to transitions in the strong absorption bands associated with C-H stretching vibration near 3.4 microm, and time-resolved fuel vapor concentration and gas temperature were determined simultaneously from the absorption at two different wavelengths. These two infrared laser wavelengths were simultaneously produced by difference-frequency generation, which combines a near-IR signal laser with two near-IR pump lasers in a periodically poled lithium niobate crystal. Injection current modulation of the pump lasers produced intensity modulation of the mid-IR, which allowed the transmitted signals from the two laser wavelengths to be detected on a single detector and separated by frequency demultiplexing. Injection current modulation produced a wavelength modulation synchronous with the intensity modulation for each of the laser wavelengths, and accurate measurement of the gasoline absorption signal required the effects of wavelength modulation to be considered. Validation experiments were conducted for a single-component hydrocarbon fuel (2,2,4-trimethyl-pentane, commonly known as iso-octane) and a gasoline blend in a heated static cell (300 < or = T < or = 600 K) and behind planar shock waves (600 < T < 1100 K) in a shock tube. With a bandwidth of 10 kHz, the measured fuel concentrations agreed within 5% RMS and the measured temperature agreed within 3% RMS to the known values. The 10 kHz bandwidth is sufficient to resolve 1 crank-angle degree at 1600 RPM.

Doppler writing and linewidth control for scanning beam interference lithography

Scanning beam interference lithography (SBIL) is a technique which is used to create large-area periodic patterns with high phase accuracy. This is accomplished by combining interference lithography and an X-Y scanning stage. We previously reported parallel scan mode in which the stage scans in a direction parallel to the interference fringes. Here we present a method called Doppler scanning. In this mode, the stage is scanned perpendicular to the interference fringes. In order to obtain high-contrast latent gratings in the exposed photoresist, several parameters must be controlled. These parameters include vibration, fringe period error, time delay (for Doppler writing), dose, beam overlap, and polarization. In this article we present results of how the time delay, fringe period error, and exposure dose effect the contrast and linewidth of our latent grating images. Furthermore, SBIL has a unique ability to read gratings in a metrology mode configuration. This article also describes how Doppler metrology...

Low-temperature-grown GaAs coplanar waveguide single-photon/two photon absorption autocorrelator

Previously, we have described a low temperature grown GaAs device that uses single-photon absorption to perform a carrier lifetime limited optical autocorrelation of picosecond optical pulses. In this article, we describe how this same device could be used to perform an autocorrelation of femtosecond optical pulses by utilizing two-photon absorption (TPA). Furthermore, we propose how to model and minimize the photocurrent’s dependence on the single-photon absorption (SPA) response of the midlevel traps. We find that the SPA response produces a distortion on the TPA autocorrelation signal at low intensities. At large peak intensities (Ipeak≈3G W cm−2), however, we find that the SPA distortion becomes nearly two orders of magnitude smaller than the TPA signal, and decreases further with increasing intensity. In our discussion, we also describe some of the tradeoffs between using a photoconductor with a large two-photon absorption coefficient and midlevel states as a TPA autocorrelator.

Photonic lantern adaptive spatial mode control in LMA fiber amplifiers

We demonstrate adaptive-spatial mode control (ASMC) in few-moded double-clad large mode area (LMA) fiber amplifiers by using an all-fiber-based photonic lantern. Three single-mode fiber inputs are used to adaptively inject the appropriate superposition of input modes in a multimode gain fiber to achieve the desired mode at the output. By actively adjusting the relative phase of the single-mode inputs, near-unity coherent combination resulting in a single fundamental mode at the output is achieved.