Internal Photodiode Research Articles

Improvement of the signal-to-noise ratio in a low power self-mixing interferometer using a coupled interferometric effect.

We present experimental results of a low-emission self-mixing interferometer that uses a coupled interferometric effect to improve the signal produced by a vibrating target. This method is intended to be useful in applications where the target is prone to be damaged by high-intensity laser sources. The beam of a Fabry-Perot laser diode is split and ∼21% of the original emission is used to measure the harmonic micro-displacements of the target using the self-mixing effect. A portion of the residual beam, which also carries the interferometric information related to the target displacement, is reinjected back into the laser cavity by means of a fixed reflector, causing a second interferometric phenomenon that improves the signal-to-noise ratio of the measurement by up to ∼13 dB. A theoretical description of the phenomena is also proposed. Further, we apply this technique to the two most common self-mixing sensing schemes: internal photodiode and junction voltage. The reported results show good agreement with theory and prove the capability of the method to enhance the SNR in SMI schemes.

Fiber laser welding of AA 5754 in the double lap-joint configuration: process development, mechanical characterization, and monitoring

Aluminum alloys are widely spread in many industrial sectors due to their desirable characteristics as low density, good formability, high specific strength, and good resistance to corrosion. Autogenous laser welding is a technology that enables the use of these materials in the industrial process due to its high repeatability, reliability, and ease of automatization. In particular, in automotive applications, Al-alloys are welded in lap-joint configurations with more than 2 layers of material. The welding condition should be monitored in order to detect the complete penetration, hence guaranteeing the appropriate weld resistance. The use of non-invasive and coaxial monitoring solutions is highly desirable for the identification of weld defects during the process. This study investigates an autogenous laser welding process and monitoring in the double lap-joint configuration of sheets of AA 5754. First, the process parameters are investigated to identify the geometrical and mechanical characteristics of the resultant welding seams at different process conditions. The employed high-brilliance 3 kW fiber laser provided the possibility of reading the back-reflected light signal from an internal photodiode. The capability of this signal to be used as a non-invasive, coaxial, and remote monitoring system in order to predict the process outcome was tested. In the experiments the back-reflected light intensity could be correlated to the weld seam width at the second interface, as well as the strength of the joint to shear. Finally, the monitoring signal behavior was demonstrated under simulated weld defect conditions. The results show that weld anomalies such as lack of penetration, misalignment, and gap formation can be sensed through the monitoring approach.

Highly integrated coupled cavity photonic crystal laser with on-chip power control on theAlGaIn/AsSb material system

We present a multi-segment photonic crystal coupled cavity laser device on GaSb with amicrostructured internal photodiode. This monolithically integrated power monitor isadded as a third segment to a coupled cavity laser and is separated from the activedevice by six rows of two-dimensional photonic crystals, acting as highly reflectingmirrors. There is no additional fabrication step needed to integrate this featureinto the coupled cavity laser, resulting in a highly integrated laser device of only800 µm length. The device with lasing wavelength around 1955 nm shows single mode emissionover a tuning range of as large as 16 nm and exhibits output powers of up to9 mW.

Highly integrated widely tunable single mode laser with on-chip power control

We report the development of widely tunable lasers monolithically integrated with aninternal photodiode separated from the active laser segments by one-dimensional photoniccrystals (PhCs). The devices cover a large total wavelength span of 42 nm in single modeemission and are tunable in wavelength steps below 1 nm whereby the outputpower can simultaneously be controlled via the internal photodiode. Discretewavelength tuning is obtained by small changes of the injection current using atwo-coupled Fabry–Perot resonators configuration. The devices with single modewavelength tunability and simultaneous power control on one single chip appearto be key enablers for future lab-on-a-chip applications in tunable diode laserspectroscopy.

Self-mixing interferometry with a laser diode: experimental considerations for sensing applications

An investigation of the effects of external reflectance and pump current on the steady state current threshold and the output power of a laser diode are presented. Back-coupled external reflectance is achieved by placing a mirror in front of the laser diode cavity. Furthermore, by using a rotating wheel, the effect of external reflectance, pump current and combination of individual contributions to frequency shifting on the laser intensity power spectrum are explored. Feedback power ratios below 0.05 and pump currents up to 2Isth (Isth, solitary threshold current) are used in these experiments. The power emitted by the two laser facets are measured through an internal (built-in package) and an external photodiode; both observations are compared. Surprisingly, when evaluating the power–current (P–I) curve for the feedback and solitary cases, a cross over is noticed at ≈1.54Isth only through the external photodiode. Nonetheless, measurements obtained with the internal photodiode agree with the theoretical steady state analysis. Moreover, it is proved that a linear combination of the individual frequency shifts holds in the power spectrum; in addition, a modulation of the carrier density by frequency-shifted optical feedback is also observed. The results from these experiments provide a practical background that ensures a correct interpretation of measurements when using this system for sensing applications.

Radiocontrolled Movement of a Robot Endoscope in the Human Gastrointestinal Tract

Background: Remote controlled movement of wireless capsule endoscope might improve diagnosis by allowing controlled examination of gastrointestinal pathology and is a pre-requisite for precise therapeutic intervention with capsule endoscopy. Aim: To develop a radio-controlled electrostimulation capsule (RESC) to propel and alter the direction of movement of a wireless capsule endoscope and to test it in man. Methods: A radio-controlled electrostimulation capsule measuring 11 × 33 mm was constructed. It featured paired bipolar electrodes inset at both ends of the capsule arranged to deliver sufficient current to cause circular muscular contraction in the gut to propel the capsule forwards and backwards. Small circuits inside the capsule were designed to receive commands and deliver pulses to move the capsule in the anesthetized pig and unanesthetized human gastrointestinal tract. 2 miniature batteries powered the device. Yellow, green and red LEDs indicated forwards or backwards electrostimulation and a strobe effect coincided with the electrostimulation frequency. An internal photodiode trigger was used to switch the device on and off. The RESC was attached to an identically sized wireless capsule endoscope by a short cable with the endoscope dome-window viewing the RESC. A command radio-control module allowed independent testing of the LEDs, could switch the electrostimulation from forwards to backwards and deliver single test pulses to tissue. A go button commanded forwards or backwards motion. A loop aerial over the abdomen delivered radio command signals. Real-time image analysis software was used to acquire capsule images using the Given-Imaging Pill-Cam. Results: The RESC worked well in animal and human trials. A “tongue-test” was developed to check the LED's and physical effect of the electrostimulation prior to swallowing. The RESC was used as a tug or engine to pull or push a Pill-Cam at will. The direction and effect of the electrostimulation signal was imaged in real-time using the wireless capsule endoscope. The combination double sausage capsule-tug combination proved easy to swallow by a human volunteer. The RESC was observed functioning in the esophagus, stomach and duodenum using both conventional endoscopes and the attached wireless capsule endoscope. Conclusion: An autonomous radiocontrolled robotic capsular endoscope with forwards and reverse movement function was tested successfully in the porcine and human gastrointestinal tract.

Liquid helium cryostat with internal fluorescence detection for x-ray absorption studies in the 2-6 keV energy region.

X-ray absorption spectroscop (XAS) in the intermediate x-ray region (2-6 keV) for dilute biological samples has been limited because of detector/flux limitations and inadequate cryogenic instrumentation. We have designed and constructed a new tailpiece/sample chamber for a commercially available liquid helium cooled cryostat which overcomes difficulties related to low fluorescence signals by using thin window materials and incorporating an internal photodiode detector. With the apparatus, XAS data at the Cl, S, and Ca K edges have been collected on frozen solutions and biological samples at temperatures down to 60 K. A separate chamber has been incorporated for collecting room-temperature spectra of standard compounds (for energy calibration purposes) which prevents contamination of the cryostat chamber and allows the sample to remain undisturbed, both important concerns for studying dilute and radiation-sensitive samples.

Monolithically integrated DWDM receiver

A grating spectrometer integrated monolithically in the InGaAsP system with a photodiode array for a dense WDM system is presented for the 1.5 μm wavelength region. The chip provides more than 30 wavelength channels with a spacing of 4 nm, a channel crosstalk of approximately −15 dB, an internal photodiode efficiency of 90% and a photodiode capacitance of 0.33 pF. The chip needs no optical adjustments. It is therefore well suited to mass production.

Grating spectrograph integrated with photodiode array in InGaAsP/InGaAs/InP

A grating spectrograph integrated monolithically for the first time in the InGaAsP system with a photodiode array for a dense WDM application is presented for the 1.5 mu m wavelength region. The chip provides 42 wavelength channels with a spacing of 4 mm, a channel crosstalk of approximately -15 dB, an internal photodiode efficiency of 90%, and a photodiode capacitance of 0.33 pF. The chip needs no optical adjustments. It is therefore well suited for mass production.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>