Optical: systems and elements – Optical amplifier – Mode locked
Reexamination Certificate
2002-03-04
2003-09-30
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Mode locked
C359S341300, C359S341320, C372S018000, C372S019000, C372S032000
Reexamination Certificate
active
06628454
ABSTRACT:
The present invention relates to a method for bidirectionally pumping an optical fibre amplifier and to a bidirectionally pumped optical amplifier. The present invention also relates to a method for stabilizing the optical emission of a laser.
BACKGROUND OF THE INVENTION
Erbium doped fibre amplifiers useful for telecommunication systems are pumped by high-power semiconductor lasers. In particular, high-power 1480 nm or 980 nm quantum well diode lasers are generally used. These lasers typically have Fabry-Perot optical cavities with no longitudinal mode selectivity and may emit over a broad wavelength range. However, the lasers must emit within an absorption band of the rare-earth ions in order to pump the amplifier. In the case of the 980 nm pumped amplifiers, the absorption band of erbium may be less than 15 nm wide, whereas the gain spectrum of a 980 nm pump laser may be as wide as 60 nm. Pump lasers must then meet stringent wavelength requirements and be immune to effects that might change the laser spectrum.
In the following of the description, a parameter called “free running wavelength” of the pump laser will indicate the operative wavelength of the laser, that is, the wavelength value of the peak of the gain spectrum of the pump laser when it is driven by a predetermined current. As the skilled in the art readily appreciates, in dependance of the driving current (or in dependance of the output power) of the laser the free running wavelength shifts, starting from a value &lgr;
th
, corresponding to the threshold current I
th
of the laser. &lgr;
th
is generally referred as “threshold wavelength”.
A known method for stabilizing the wavelength emission of a 980 nm pump laser is the use of a low reflectivity grating coupled to the output, anti-reflection coated, low-reflectivity facet of the laser. See, for example, a first article of Giles et al., Reflection-Induced Changes in the Optical Spectra of 980-nm QW Lasers,
IEEE Photonics Technology Letters
, Vol.6, No.8, August 1994. According to Giles et al.'s first article, the use uf the grating allows the reduction of the sensitivity of the pump laser to weak reflections that affect the laser emission spectrum.
In a second article (Giles et al., Simultaneous Wavelength-Stabilization of 980-nm Pump Lasers,
IEEE Photonics Technology Letters
, Vol.6, No.8, August 1994), the same authors disclose the simultaneous wavelength-locking and stabilization of three 980-nm pump lasers, connected to the input ports of a 4×4 fibre star coupler, through reflection from a single narrow-band fibre grating connected to one output port. According to Giles et al.'s second article introduction, injection-locking is a means for stabilizing laser sources, but may not be practical for the compact, low-cost sources required to pump the erbium doped fibre amplifier.
Injection-locking is a well known technique used to achieve single-longitudinal-mode operation of a multi-longitudinal-mode semiconductor laser by suppressing the side modes with continuous wave single-longitudinal-mode master laser injection phase-matched with the output emission of the injected laser and of a wavelength comprised in a locking bandwidth that ranges from 100 MHz to some GHz around the output emission &lgr;. This technique is presently used in optical systems for precisely selecting wavelength emission of laser transmitters in a bandwidth of about 100 MHz; it has also been proposed for reducing frequency-chirped dynamic linewidth in directly modulated single-longitudinal-mode semiconductor laser transmitters (see for example C. Lin, J. K. Andersen, Frequency chirp reduction in a 2.2 Gbit/s directly modulated InGaAsP semiconductor laser by cw injection,
Electronics Letters
, Jan. 17, 1985, Vol.21 No.2).
FIG.1
shows a configuration of a known bidirectionally pumped optical fibre amplifier
100
, comprising an amplifying doped optical fibre section
101
, for example an erbium doped amplifying fibre, pump lasers
102
,
103
, WDM couplers
104
,
105
, optical isolators
106
,
107
for light signals, input and output terminals
108
,
109
. A signal light is launched in the amplifier
100
through the input terminal
108
, travels along the doped fibre section
101
to be amplified therein and exits through the output terminal
109
. Suitable energy for amplification is provided by pump lasers
102
,
103
, which couple pump light to the doped fibre
101
through WDM couplers
104
,
105
. In particular, pump light from laser
102
is launched co-directionally in the doped fibre
101
, that is in the same direction of the signal light, whereas pump light from laser
103
is launched counter-directionally, that is, in the opposite direction with respect to the signal light. For an erbium doped fibre amplifier, pump lasers
102
,
103
may emit light whose wavelength is comprised in a pumping band centered around 980 nm or 1480 nm.
Herein and in the following of the description, the expressions “co-directionally”, “counter-directionally”, “co-propagating”, “counter-propagating” will be always referred to the propagation direction of the signal light.
The configuration shown in
FIG. 1
has a problem in that the residual pump light from each pump laser, not fully absorbed by the amplifying fibre, is injected into the opposite pump laser, which can result in optical instabilities and fluctuations in amplification of the optical signal.
It is known that such instability can be avoided by placing an isolator on the optical path of each of the pumps.
In patent U.S. Pat. No. 5,640,268 to Alcatel N.V. a solution is addressed to this problem. According to the '268 patent, each pump injection fibre includes a photorefractive pump filter constituting part of the resonant cavity of the associated pump laser, the two pump filters being mutually different to give rise to an offset between the two pumping bands. The two pumping bands are preferably offset by several nanometers. The pump filters are photorefractive gratings having a determined pitch and thus a determined central wavelength for reflection: the use of such a grating makes it possible simultaneously to reduce the width of the pumping band and to position said band more accurately within the spectrum.
Applicant has experimentally verified that in a configuration according to the '268 patent, if the wavelength emitted by the pump lasers is within the 980 nm pumping band the offset between the two pumping bands should be greater than 15 nm, in order to avoid instabilities due to residual pump injection. Since for an erbium doped fibre amplifier, the pumping band centered around 980 nm is only 10-15 nm wide, an offset between the two photorefractive filters of 15 nm or more would lead one of the pump wavelenghts to be nearly out of the pumping band of erbium, considerably reducing the bidirectional pumping efficiency.
SUMMARY OF THE INVENTION
Applicant has found that a pump laser, even without a stabilizing grating, can have a stable optical emission if it is injected by an external radiation having a wavelength close to the free running wavelength of the injected laser and having a sufficiently high power. A locking of the optical emission of the injected laser around the wavelength of the injection takes place. The useful “locking bandwidth”, that is, the useful difference between the injected wavelength and the free running wavelength of the injected laser may range up to several nanometers. Stability of the optical emission means that at least 80% of the power emitted by the injected laser is comprised in a wavelength range of about 2 nm around the wavelength of the injection.
Applicant has also found that in a bidirectionally pumped optical amplifier a pump residual due to a pump radiation not absorbed in an active fibre, said pump radiation coming from a first pump laser used with a stabilizing grating, may have a sufficient power for stably locking the optical emission of a second pump laser, used without a stabilizing grating. This can lead to substantial elimination of the optical instabilit
Balsamo Stefano
Ghislotti Giorgio
Trezzi Fiorenzo
Corning Oti SpA
Moskowitz Nelson
Short Svetlana Z.
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