100%" bgcolor=white>
Photodigm Photodigm

APPLICATION NOTE: PHOTODIGM SPECTROSCOPY SERIES LASERS
Rubidium Absorption Spectroscopy

Introduction
Ultra-high Photodigm/' >resolution laser spectroscopy of the rubidium
atom at 780 nm has numerous applications in sensing,
metrology, and aerospace. Despite these opportunities,
commercial product development has been hindered by
unstable sources of supply of precision, reliable
semiconductor lasers at this wavelength. Photodigm has
introduced a 780 nm high-power distributed Bragg
reflector (DBR) laser diode in its Spectroscopy Series
that has the characteristics necessary to meet this
growing commercial need. These devices are fabricated
using Photodigms proprietary technology encompassing
grating design methodology, single-growth epitaxy, and
ridge waveguide process with holographic gratings.
Typical output power of these DBR lasers is 60 mW.
With a side-mode suppression ratio exceeding 45 dB
over the specified range, they are ideal for spectroscopic
applications.
This robust process ensures high yield at the target
wavelength and high reliability with long lifetime. As a
result both atomic spectroscopists and instrument
manufacturers will be able to confidently move forward
with their programs.
Experimental Set-up
The optical part of the experimental set-up consist of a
beam splitter BS, two photodetectors PD1 and PD2, a
100 mm long glass cell filled with natural rubidium, and
Photodigms 780 nm high-power DBR laser diode (see
Fig. 1). An oscilloscope measures the ratio between the
two signals.
Fig. 1: Schematic of experimental set-up.

A constant current of 78 mA and a small 10 kHz saw-
tooth modulation of a few mA simultaneously drove the
DBR laser diode. The heat sink temperature was tuned
until the absorption lines were centered on the scope.
Thermal and current tuning are 0.6
Å/°C and
0.026
Å/mA, i.e. 30
GHz/°C and 1.3
GHz/mA,
respectively.
Results
Natural rubidium consists of mainly two isotopes:
85
Rb
(72.2%) and
87
Rb (27.8%). Focusing on the D2
transition, both isotopes absorb around 780.024 nm but
their corresponding hyperfine splitting frequencies are
different. The ground state of either isotope splits into
two hyperfine levels. The exited state hyperfine splitting
is masked by the ~500 MHz Doppler broadening so that
individual levels are indistinguishable by simple
spectroscopy techniques. Thus, the expected transitions
have ground state hyperfine detuning of 6.834 GHz and
3.035 GHz for the isotopes
87
Rb and
85
Rb, respectively.
Figure 2 presents the absorption spectrum of natural
rubidium monitored with the mentioned set-up. The four
absorption peaks, two resulting from each isotope, are
clearly visible and distinguished.

Fig. 2: Absorption spectrum of
85
Rb and
87
Rb
recorded using Photodigm 780 nm DBR laser
tuned over a 7 mA range.

The linewidth of the used DBR laser diode is below
10 MHz and compared to the Doppler broadening of the
transitions almost two orders of magnitude smaller.
Rb cell
BS
DBR
PD2
PD1
Laser
Temperature
Controller Laser Lifetime
A cell of six DBR laser diodes has been monitored over
time, when running in constant power mode. The rated
output power was 60 mW and the selected temperature
was fixed at 25°C, 45°C and 65°C (see Fig. 3).

Fig. 3: Injection current against operation hours
(ongoing test, as of March 3, 2008).
Initial lifetime data were estimated assuming a current
limit of 1.5
× the initial current value. Table I presents
mean-time-to-fail MTTF values for three different
temperatures.









Table I: Projected device lifetime based on operation
hours.
Further 780 nm DBR Laser Applications
The PD780DBR-1-LD has many uses within the bio-
medical, sensing, metrology, and aerospace markets.
Atom cooling and trapping applications benefit greatly
from the high power and frequency stability found in
Photodigms proprietary DBR structure. Data storage
such as optical ROM, holographic storage, and disc
mastering are all uses for 780nm DBR laser.
One of the highest growth areas for this single-
frequency, high power DBR lasers are in navigation and
spectroscopy.
Photodigms lasers can simplify instrumentation design
by reducing or eliminating the need for amplification or
frequency stabilization components. This can result in
dramatic cost savings for OEMs. Contact Photodigm for
a laser solution, tailored to your application.

About Photodigm
Founded in 2000, Photodigm produces
semiconductor diode lasers for the precision
instruments and defense markets using its
proprietary distributed Bragg reflector
technology. By focusing on high power, single
frequency diode lasers in the industrially
significant 780 nm to 1100 nm spectral range,
Photodigm is bringing unique capability to its
customers, who have not had access to a stable
supply of product.
Photodigm lasers are offered at 780, 920, 976,
1064, and 1083 nm wavelengths as standard
products free space optical and fiber pigtailed
packages. The Spectroscopy Series offers the
target wavelength +/- 0.5 nm, and the Power
Series at wavelength +/- 3 nm.
Custom wavelengths and applications support
for spectroscopy, non-linear optics, and fiber
amplifier seeding are available from
Photodigm.



For further information please contact:


Sales and Marketing
Photodigm, Inc.
1155 E. Collins Blvd.
Suite 200
Richardson, TX 75081
Tel. 972.235.7584
Fax 972.235.1609
sales@photodigm.com

65
45
25
Temp
(ºC)
11,260
20,380
41,560
MTTF
(1.5 x start current [hrs])