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Tunable Diode Lasers Tunable Diode Lasers


Diode is also called crystal diodes, referred to as diode; it only to send current in one direction of electronic components. It is a bonding with a part number of two terminal devices with in accordance with the direction of the applied voltage so that current flow or not flow of nature.

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									Tunable Diode Lasers
Tunable diode lasers are increasing
their market penetration via rapid
evolution — a new generation
of products every year or so.

Bob Shine and
Tim Day, New Focus

At the recent Photonics West and
Optical Fiber Conferences, compa-
nies presented many new products,
ranging from mechanical products to
instruments to complex laser sys-
tems. As a product engineer, there is
nothing quite like the feeling of
standing in the booth, talking to po-
tential customers about the product
You’ve designed or built. But the real     be described in greater
success happens when customers buy         detail in upcoming sec-
the product                                tions.)
    Customers decide to buy for a             The technical cap-
number of reasons. The product may         abilities and features of
let them do something that was dif-        the tunable diode laser
ficult or even impossible before. Or       played a major role in
the product might make their experi-       the success of the pro-
ments simpler by creating a better-        duct. But the ease-of
designed or easier-to-use instrument       use of the product also
than was previously available. For this    contributed to its suc-
to happen, the product must clearly        cess. For example, a
satisfy a real need of the customer,       room-temperature,      2-
the product should have some sus-          µm tunable diode laser
tainable competitive advantage over        was recently used by re-
other companies’ offerings, and the        searchers at Stanford University and     Figure 1. (top) The Vortex laser (New Fo-
company should be able to make a           Focused Research to obtain a survey      cus’ newest) is built to the customer’s
profit by selling the product. This last   spectrum of CO 2 in a matter of          specific wavelength specification and off-
item is important since the product        minutes. With this type of technolo-     ers robust, narrow-linewidth source for
                                           gy these researchers can imagine a       atomec spectroscopy, environmental
can survive only if it can be justified                                             monitoring, and metrology applications.
economically both to the customer          very sensitive diode-laser-based sen-
and the manufacturer.                      sor for combustion monitoring.           Figure 2. (below) Atomic absorption
    The tunable diode laser is a pro-                                               monitor based on a New Focus Vortex
duct that has satisfied all of the         Some History                             tunable laser and built by Orca Photonics.
                                           Before the tunable diode laser was       This system was designed to monitor bari-
above requirements to become a
                                           introduced, the workhorse of the         um deposition rates and has been de-
successful product. Since the first                                                 ployed with an industrial customer.
commercial introduction of this type       tunable laser market was the liquid
of laser about five years ago, it has      dye laser. This laser was developed in
found numerous applications, ranging       the 1960s and was used in a number       the dye laser in the 600- to 1000-nm
from telecommunications to spec-           of fundamental discoveries in spec-      range. But the Ti:sapphire laser still
troscopy to metrology. While each          troscopy. Different dyes allowed us-     required an expensive pump laser and
of these applications places slightly      ers to cover a wide spectral region.     water cooling, yet did not directly
different requirements on the laser        However, the dye laser had a number      offer the wavelength diversity
and the user interface, each of them       of drawbacks, such as the need for       (roughly 350 to 1200 nm) of the dye
requires a narrow-linewidth, continu-      an expensive pump laser and the in-      laser.
ously tunable, reliable source. (As ex-    convenience of using and changing            At the same time, semiconductor
amples, Figures 1 and 2 depict two         the liquid dyes. The Ti:sapphire laser   diode lasers were also widely avail-
somewhat different system that will        was developed in the late 1980s and      able. A diode laser can be tuned over
                                           offered a solid-state replacement for    its large gain spectrum by adjusting
                                                           angle of the mirror which se-    to 2.06 µm with an output power as
                                                           lects a unique diffracted wa-    high as 18 mW. The tuning curve for
                                                           velength. The reflected zero     this laser is shown in Figure 4.
                                                           order from the diffraction          The 2-µm region and beyond Is of
                                                           grating has constant direction   particular interest in environmental
                                                           and forms the output beam.       monitoring, since these wavelengths
                                                           These external cavity designs    allow access to strong ground-state
                                                           yield continuous tuning over     vibrational overtones of many major
                                                           the wide gain curve of the       pollutants. Professor Ron Hanson
                                                           diode laser element with a       and his group at the High Tempera-
                                                           very narrow linewidth!           ture Gasdynamics, Laboratory in the
                                                            The tuning range depends on     Mechanical Engineering Department
                                                           the gain element used in the     at Stanford University (Palo Alto,
 Figure 3. Schematic drawing of a Littmann-Metcalf
 laser cavity. The angle of the tuning mirror selects
                                                           cavity; at 630 nm the tuning     Calif) have been investigating flame
 the output wavelength and the diffrection grating         range is 10 nm, while at 1550    dynamics in combustion chambers
 acts as a frequency-selective output coupler. The         nm the tuning range can be       for a number of years. Their aim is to
 location of the pivot point is critical to obtaining      greater than 70 nm. In both      produce a compact, reliable sensor
 continuous tuning without mode hops.                      cases, the linewidth is less     for CO2 and other combustion gases
                                                           than 300 kHz. The inherent       to measure the efficiency of burn
its operating temperature. However, efficiencies in the already mature                      chambers and incinerators. They have
because of the semiconductor’s in- diode laser market helped make the                       focused on sensors based on diode
herently broad spectrum, more than external-cavity diode laser an attrac-                   lasers due to the robustness, reason-
one mode will often operate simulta-               tive replacement for conventional        able cost, and relative ease-of-use of
neously. This produces multiple out- dye and solid-state tunable technol-                   these lasers. Because of this focus on
put wavelengths and, therefore, a ogies. As a result, these lasers have                     compact sources, their work has al-
broad spectral linewidth.                          quickly found use in the applications    ways been dictated by the available
                                                   mentioned above.                         wavelengths of the diode lasers. In
Adapting The Diode Laser                                                                    the past, this constraint has limited
Adding an external optical cavity             Some Recent Applications                      their sensors to detection of the rel-
forces the diode laser to operate in a        One of the benefits of the tunable            atively weak transitions associated
single longitudinal mode by creating a        diode laser is the wavelength diversi-        with vibration-rotation bands. For
wavelength-dependent loss within the          ty that can be obtained. Almost any           comparison, the CO2 linestrengths in
laser cavity. In practice, this cavity        wavelength that is available in a semi-       the 2 µm region are 70 times stronger
can be either a Littman-Metcalf or a          conductor diode laser can be made             than at 1.58 µm. This group has re-
Littrow design — two cavity designs           into a tunable diode laser. The most          cently collaborated with Focused Re-
widely used in dye lasers. Both of            recent example is in the 2-µm region.         search and has used a 2-µm tunable
these cavities consist of a diode laser       In a partnership with Focused Re-             diode laser to obtain survey spectra
gain element with one facet antire-           search (a subsidiary of New Focus),           of CO2 and H 2O at various temper-
flection (AR) coated for very low             researchers at the Sarnoff Corpora-           atures and pressures. A representa-
(<10 ) reflectivity. The output from          tion (Princeton, N.J.) have demon-            tive CO 2 survey spectrum is shown in
the AR-coated facet is collimated             strated strained InGaAs/InP quantum           Figure 5. From these survey spectra,
and directed onto a highly dispersive         well lam with center wavelengths near         they have selected a strong CO2 ab-
diffraction grating,                          2.02      µm.
    In the Littrow cavity, the angle of       These semi-
incidence is such that the beam is            conductor
diffracted back on itself. The grating        diode lasers
therefore serves as one mirror in the         were       AR
cavity; tuning is achieved by control-        coated and
ling the angle of the grating. In the         placed in a
more       common      Littman-Metcalf        specially de-
design, shown in Figure 3, the grating        signed exter-
diffracts the light toward a tuning           nal cavity by
mirror, which reflects the desired wa-        a team at
velength back towards the grating             Focused Re-
and gain medium. This double-pass             search. This
scheme, coupled with the grazing in-          laser operat-
cidence on the grating, results in a          ed at room
very narrow spectral passband, and            temperature
therefore excellent wavelength sen-           and demon-
sitivity, without the use of additional       strated con- Figure 4. Tuning range of an external-cavity tunable diode laser using
intracavity filters such as an etalon.        tinuous tun- a strained InGaAs/InP quantum well as the gain element. This 2-µm
Tuning is achieved by adjusting the           ing from 1.96 laser was operated at room temperature.
                                                                                      offers species-specific measurement
                                                                                      of the deposition process, including
                                                                                      flux velocities and spatial and tem-
                                                                                      poral homogeneity. In an atomic ab-
                                                                                      sorption monitor, the vapor absorbs
                                                                                      light at the wavelength corresponding
                                                                                      to an atomic transition; measurement
                                                                                      of the incident and transmitted light
                                                                                      yields the evaporation rate. While
                                                                                      tunable diode lasers are more expen-
                                                                                      sive than other light sources such as
                                                                                      hollow cathode lamps, the higher
                                                                                      spectral intensity translates into
                                                                                      faster data acquisition. In addition,
                                                                                      the laser’s modulation capability is
                                                                                      useful for the development of a
                                                                                      more sensitive, drift-free sensor
                                                                                      (Figure 2). The narrow linewidth of a
         Figure 5. Measured survey spectra of CO2 between 4866 cm-1                   laser source enables spatial, tempo-
         (2.055 µm) and 5118 cm -1 (1.954 µm) taken using a room                      ral, and velocity mapping of the flux
         temperature external cavity diode laser.                                     and creates a more complete under-
                                                                                      standing of the deposition process.
sorption band relatively free from         imagine a metrology application            This improved understanding can
background absorptions that offers         needing a stable, narrow-linewidth         then be used to increase the process
the best opportunity for sensitive         source but with a narrow tuning range      yield.
CO 2 measurements.                         that would allow FM measurements               While the pace of development in
   As mentioned briefly earlier, each      to be made, something not possible         the tunable laser field has been quite
application tends to require a differ-     with a HeNe laser.                         fast, with a new laser system avail-
ent set of features. It is tempting to        To satisfy such needs, New Focus        able almost every year, each new
continue adding features to a pro-         uses the same Littman-Metcalf exter-       generation has offered customers
duct in an attempt to satisfy every-       nal cavity design as in their other tun-   additional benefits. Additional wave-
one. But this approach often leads         able diode lawn but removes the            lengths such as the 2-µm tunable
to an overly complex and expensive         coarse tuning feature.                     diode laser and products focused on
instrument that satisfies no one. In          One example of an industrial ap-        customers needs have allowed cus-
many instances, the product with the       plication of this laser is the collabo-    tomers to do what they could not
fewer features is the better product.      rative work of Focused Research and        have done before and provided sim-
   This is the approach New Focus          Orca Photonics Systems. Physical va-       pler, easier-to-use tools for them.
decided to use when designing the          por deposition techniques such as          This focus on customer needs that
newest tunable diode laser; the Vor-       electron beam, sputtering and mo-          continue to expand the market for
tex (Figure 1). There are a number of      lecular-beam epitaxy (MBE) are criti-      the already successful tunable diode
applications where the full tuning         cal to the manufacture of semicon-         laser.
range of the tunable diode laser is        ductor devices, high-performance al-
not needed or used. Examples in-           loys, and high-temperature supercon-
clude studying a single atom or mol-       ductors. The product yield depends
ecule, such as rubidium, with a well-      on precise control of the flux of the      Bob Shine is the marketing manager and
known atomic transition. Or, as in         different elements during manufac-         Tim Day is the vice-president of engineer-
the work described above, an opti-         ture. Historically, techniques such as     ing at New Focus Inc, 2630 Walsh Avenue,
                                                                                      Santa Clara CA 95051. Phone (408) 980-
mum wavelength can be selected af-         quartz crystal monitors, quadrupole
                                                                                      8088; fax (408) 980-8883.
ter obtaining an initial survey spec-      mass spectrometers, and ion gauges
trum with a broadly tunable laser and      have been used in process control          Originally published in Lasers &
identifying a wavelength free from         for such applications.                     Optronics, March 1998, pp 13-14.
background sources. Or you could              Atomic absorption monitoring

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