We report the experimental and theoretical results for high reflectivity (>99% at 850 nm) AlAs/AlGaAs distributed Bragg reflectors (DBR) for vertical cavity surface emitting semiconductor lasers (VCSEL). It is investigated that up to ± 10% deviations in thickness distribution of DBR layers negligibly affects the reflected bandwidth if the distribution is random. However, ascending/ descending order in thickness distribution beyond ±5% results in rapid shrinking of the reflected bandwidth with >99% reflectivity.
An automatic and accurate technique
for angular positioning of mask with
respect to wafer is reported.. Alignment
marks are in the form of gratings and the
moire signal is obtained by the relative
displacement between the gratings. The
higher slope region of the moire signal
is used to obtain higher sensitivity and
better position control accuracy. The
experiments are performed with 25 micrometer pitch gratings and Piezo-
Electric Transducer is used for the
angular displacement. The angular
-7
accuracy of the order of 2 x 10 radian is
reported with a time constant of 0.2 sec.
In the recent years, use of moire
interference technique for mask to wafer
alignment has attracted much attention
especially in VLSI fabrication technology
where a highly accurate linear and
angular positioning is involved. In this
technique the alignment marks are in the
form of gratings.
A laser beam is passed normally
through a mask grating and reflected from
the wafer grating. Intensity variation in
the reflection mode due to relative
angular rotation is detected by a
photodetector and converted into the
electrical signal, known as moire signal.
Small angle measurement using a modified moiré interference technique is reported. The range of angle measurement for 25-μm pitch gratings is about ± 25 μrad and the accuracy of measurement is of the order of 0.2 μrad with a time constant of 0.1 s. A piezoelectric transducer (PZT) is used for angular displacement. The angular sensitivity under the present experimental conditions is found to be of the order of 21.8 mV/μrad.
A new approach for small angle measurement by moiré interference technique is reported. Moiré signal is obtained by a pair of parallel Ronchi ruling gratings in the reflection mode. A relative rotation between the two gratings gives rise to- the intensity variation known as the moiré signal. The present approach for small angle measurement utilizes the region of higher slope of the moiré signal to get very high accuracy. The moiré signal is amplified and its inversion is obtained electronically. The difference of the moiré and its inverted signal with suitable dc shift is obtained by differencial amplifier and used for the small angle measurement. The present technique is successfully used for small angle measurement with the accuracy of the order of 10−7 radian with a time constant 0.1 sec. The accuracy is found to be dependent on the pitch of the grating, signal to noise ratio of the system and the response time. The experiments are carried out using a pair of 25μm pitch gratings. Piezo-electric transducer (PZT) is used to drive one of the gratings and the angular sensitivity of the order of 500 mV/ micro radian is observed.
We report measurements of the statistical properties associated with the intensity fluctuations of a He-Ne laser propagated through a laboratory simulated atmospheric turbulence as well as through the open air atmospheric turbulence region. From these measurements the probability density function representative of the random process was identified by comparing the measured parameters with those of standard theoretical models. Photon counting techniques have been used in all our measurements.
Experimental results for autocorrelation function of scattered laser light from rotating ground glass are reported, which set aside the controversy regarding its speed and angular dependence.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.