Low noise and wide power range laser source

Details Low noise and wide power range laser source Low noise and wide power range laser source

1998-12-08

2002-06-11

Ip, Paul (Department: 2828)

Coherent light generators

Particular resonant cavity

Specified output coupling device

C372S097000, C372S099000, C372S102000

Reexamination Certificate

active

06404798

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to laser sources and further to equipments for measuring optical components.
BACKGROUND OF THE INVENTION
In optical communication networks, information is generally transmitted by optical fibers from a stimulus, e.g. a laser diode, to an optical receiver, e.g. a photo diode. These may not only be point-to-point links but may also provide a complex network structure which generates the need for optical components for data routing, adding, dropping and switching.
To increase the transmission capacity, several communication channels are normally used simultaneously. In principle, this can be realized by separating the channels by providing time multiplexing or centering channels at different wavelengths. The latter principle is also known as Wavelength-Division-Multiplexing (WDM) and is becoming increasingly important. A state of the art WDM system has about 20 channels separated by 0.8 nm in a wavelength range of around 1550 nm. First research work is already done to increase the amount of channels by reducing the channel spacing down to 0.2 nm and therefore increase the transmission capacity by about four times.
One of the problems by using WDM is the interference (cross talk) of the communication channels. To avoid interference, the used optical components need to exhibit a high wavelength dependent transmission characteristics, that is, e.g., a WDM cross-connect switch with a transmission dynamic of up to 30 dB over tenths of a nanometer. A known complex and expensive measurement setup for characterizing this kind of optical components is based on a tunable laser source, a wavelength meter, a tracking filter and an optical power meter (cf., e.g., in AFiber optic test and measurement@ by Dennis Derickson, ISBN 0-13-53480-5, page 358 ff.).
In general, the signal to total noise ratio of a laser source (e.g. a tunable laser source as depicted in the above mentioned book by Dennis Derickson on page 360) limits its applications where high transmission dynamics characteristics have to be measured, e.g., in case of a notch-filter with a high signal suppression, such as a fiber grating, where the back noise (SSE, ASE) of the laser source determines the measured suppression of a signal positioned at a center wavelength of the filter (cf. FIG.
6
).
A solution to improve the signal-to-noise ratio of a laser system is to provide a filter in combination with a broadband receiver or an optical spectrum analyzer. To ensure also the wavelength accuracy of the measurement which is very important in WDM systems with narrow channel spacing, also known as Dense Wavelength Division Multiplexing (DWDM), an external wavelength meter has to be used. For all these setups an additional controller plus software is needed for synchronizing and data capturing.
JP-A-06 140717, Lewis L. L. in “Low noise laser for optically pumped cesium standards” (proceedings of the annual frequency control symposium, Denver, May 31-Jun. 2, 1989, no. Symp. 43, May 31, 1989, Institute of electrical and electronics engineers, pages 151-157, XP000089353), and Boshier M. G. et al. in “External-Cavity frequency stabilization of visible and infrared semiconductor lasers for high resolution spectroscopy” (Optics communications, vol. 85, no. 4, Sept. 15, 1991, pages 355-359, XP000226852) disclose laser systems with a beam splitter provided in the external cavity.
FIG. 1
shows in principle a laser source
5
according to those art documents.
A laser gain medium or amplifier
10
provides a first facet
20
which is low reflective and a second facet
30
which is high reflective. The first facet
20
emits a laser beam
50
into an external cavity of the laser source
5
. A collimating lens
60
collimates the laser beam
50
to a beam splitter
65
splitting the laser beam
50
into a part
50
′ and a part
67
. The part
50
′ of the laser beam
50
is directed to an optical grating
70
as a wavelength dependent mirror. The optical grating
70
″ diffracts the incident beam
50
′ and a wavelength separated beam
50
″ is directed back towards the beam splitter
65
. The angle of the optical grating
70
with respect to the beam
50
″ depends on the wavelength to be selected. The optical grating
70
together with the facet
30
of the semiconductor amplifier
10
define the optical resonator of the laser source
5
. The beam splitter
65
splits up the returning beam
50
″ into a beam
50
′″ towards the gain medium
10
and a beam
80
. The laser system
5
provides as output signals the laser beams
67
and
80
, coupled out respectively from the beam splitter
65
. The output beam
80
can be coupled into a fiber
90
, e.g., by means of an optical lens
100
.
Jeffrey Bernstein et al. in “Oscillator design improves dye-laser performance” (Laser Focus World, September 1995, pages 117 ff.) discloses that the laser beam
80
, which is substantially coupled out directly after wavelength selection by the optical grating
70
provides an improved lower signal-to-noise ratio output with respect to the output beam
67
.
In modem laser applications, in particular for measuring purposes e.g. for measuring modem optical components for DWDM, it becomes increasingly important to provide flexible laser systems offering a wide range of laser signals from high power signals to low noise signals. Although the output.
67
, coupled out directly the gain medium
10
in the laser system
5
of
FIG. 1
, provides a possibility for a higher power output with regard to the output
80
, the output
67
finds a power limitation in the beam splitter
65
. Since the beam splitter
65
couples out as well the beam
80
as the beam
67
necessarily with the same coupling-out-ratio, or in other words, since the beam splitter
65
couples out the power of beam
67
or
80
, a certain tradeoff between the possible power to be coupled out and the resonator conditions for an efficient power amplification by the external cavity has to be found. This, however, limits the possible applications of the laser systems and generally renders them to be not sufficiently flexible.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to provide a flexible laser system offering a wide range of laser signals from in particular high power signals to low noise signals. It is a further object to provide a low cost and smaller flexible laser system. This object is solved by independent claims
1
or
2
. It is another object to provide a low cost measuring setup for determining high transmission dynamics characteristics. This object is solved by independent claim
8
. Preferred embodiments are shown by the dependent claims.
The invention is based on a laser source with an optical resonator. The laser source comprises a laser gain medium, e.g. a semiconductor and/or fiber amplifier, for emitting a laser beam, a wavelength dependent mirror for receiving the laser beam and reflecting back a wavelength separated laser beam, and a beam splitter for dividing the wavelength separated laser beam into a feedback beam directed toward the semiconductor amplifier and an output beam to be coupled out of the optical resonator of the laser source, preferably into an optical fiber.
According to a first aspect of the invention, the laser gain medium comprises a second facet that is partly reflective, so that the second facet emits a second output beam of the laser source, which is preferably coupled into a second optical fiber. The second output beam provides a significantly higher output power than the beam
67
as depicted in
FIG. 1
, since it is coupled out directly from the gain medium. Additionally, it has been shown that the second output beam provides an improved signal-to-noise ratio with respect to the beam
67
, in particular when the gain medium comes to a saturation condition/state. Further more, the second output beam is only influenced by the second facet and not by any other component such as the beam splitter or the like. This leads to an i

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