Coherent light generators – Particular beam control device – Modulation
Reexamination Certificate
2001-04-19
2004-09-14
Wong, Don (Department: 2828)
Coherent light generators
Particular beam control device
Modulation
C372S026000, C372S083000, C372S084000
Reexamination Certificate
active
06792011
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to frequency modulation of optical signals, and, more particularly, a frequency modulated laser that can be modulated at almost unlimited modulation rates.
BACKGROUND OF THE INVENTION
Modulation of information onto optical signals is well-known in the art. One modulation technique used for modulating optical signals is frequency modulation, which allows an improvement in Signal to Noise Ratio (SNR) by utilizing a transmission bandwidth that is larger than the signal bandwidth. Of key interest in frequency modulating optical signals, is the wide bandwidth available for frequency modulation due to the high carrier frequency in optical signals used for information transmission. Typically, communication systems offer bandwidths that are about 10% of the carrier frequency. A 1.5 &mgr;m optical system, with a carrier frequency of 200,000 GHz, has, therefore, 20,000 GHz of instantaneous bandwidth. This provides the ability to modulate data at extremely large data rates onto the optical signal.
To take advantage of the wide bandwidths available at optical frequencies, apparatus and techniques must be used that provide for high modulation bandwidths. One approach is to couple an optical laser source with an external optical frequency modulator (i.e., a phase modulator with an appropriate driver). External modulators may require complex traveling wave structures that match the propagation velocities of the modulating RF field and the optical carrier. Another, less complex, approach is to directly frequency modulate the optical laser source.
Frequency-modulated lasers are well-known in the art. A conventional technique for providing a frequency-modulated semiconductor laser is to directly modulate the drive current of a Distributed Feedback (DFB) laser. Modulation of the drive current provides about 150 to 300 MHz of frequency shift per milliamp of drive current for commercial DFB lasers, yielding a maximum frequency swing of 10-20 GHz. This approach has two significant drawbacks. First, modulation of the drive current produces significant amplitude modulation, which is undesirable for FM communication systems. Second, an upper modulation rate of only about 20 GHz is achievable, due to semiconductor device limitations.
Another approach for providing a frequency-modulated laser is shown in FIG.
1
. In
FIG. 1
, a laser cavity
10
, as defined by the partial mirrors
15
at each end, comprises an optical gain section
11
combined with a phase modulator
13
of finite length. The partial mirrors
15
provide the required reflectivity for the lasing effect. The gain section
11
is typically a laser diode and the phase modulator
13
is a section in which the index of refraction is changed by the application of either a voltage (Stark Effect) or an injection current. The inclusion of the phase modulator
13
within the laser cavity
10
converts any phase modulation produced by the phase modulator into frequency modulation. It does this by changing the effective optical cavity length, which changes the laser oscillation frequency accordingly.
Semiconductor lasers which provide for an active region and a phase modulation region are described by Emura et al., in U.S. Pat. No. 5,325,382, “Method and Electrode Arrangement For Inducing Flat Frequency Response In Semiconductor Laser,” issued Jun. 28, 1994, and Kawamura, in U.S. Pat. No. 5,481,559, “Light Modulator Integrated Light-Emitting Device And Method Of Manufacturing The Same,” issued Jan. 2, 1996. Emura et al. describe the application of a modulation current to both the active region and the phase modulation region to achieve a relatively flat frequency modulation response. Kawamura describes the provision of a ground electrode between the active region and the phase modulation region to isolate the electric field in the active region from the modulating electric field in the phase modulation region.
In the frequency-modulated laser shown in
FIG. 1
, the instantaneous frequency of the laser is given by:
f
(
t
)
=
1
2
π
ⅆ
φ
ⅆ
t
=
f
0
+
Φ
0
m
2
π
T
c
sin
c
(
ω
T
p
/
2
)
sin
c
(
ω
T
c
/
2
)
sin
(
ω
m
t
+
ϑ
)
(
1
)
where &ohgr; is the modulation frequency, T
p
is the round-trip transit time for light in the phase modulator, T
c
is the total round-trip transit time (phase modulator section+gain section) and &PHgr;
0m
is given by:
Φ
0
m
=
2
π
λ
Δ
n
0
2
L
p
(
2
)
where &Dgr;n
0
is the electrically-induced change in the index of refraction, L
p
is the length of the phase modulator, and &lgr; is the wavelength of the light generated by the laser.
Because T
c
will always be larger than T
p
, the term in the denominator will approach zero first, and produce the resonant response shown in FIG.
2
. The resonant frequency of 1/T
c
prevents the use of this prior art modulator for applying frequency modulation in the vicinity of the resonant frequency, and thus limits the bandwidth of the modulator. Hence, this prior art modulator is not suitable for high-fidelity, high bandwidth analog optical links.
If the total round-trip travel time for the laser cavity, T
c
in equation 1 above, were identically equal to the round-trip travel time within the phase modulation section, T
p
in equation 1 above, the two sinc terms in equation 1 would cancel, and the frequency response would be flat. Creation of a frequency-modulated laser where T
c
equals T
p
would provide a FM laser with a perfectly flat response over an almost infinite bandwidth.
There are two ways to make T
c
equal T
p
, one asymptotic, one absolute. The asymptotic approach is to make the length of the gain section very small compared to that of the phase modulation section, so that T
c
asymptotically approaches T
p
. The absolute approach is to make the gain and phase modulation sections coincident, so that the length of the section in which no phase modulation occurs is zero.
One structure in which the gain and phase modulation sections are longitudinally coincident is the twin-guide structure developed by Amman et al, as shown in the laser device
300
of FIG.
3
. In spite of its name, the twin-guide structure is actually a single-guide device. The laser device is split into two sections, one being the phase modulation section
310
or tuning zone, the other the gain section
320
or active zone. The relative indices of these two sections are both higher than that of the outside cladding regions
330
, so that the optical mode is confined in the center, just as it is in an optical fiber. A thin doped center layer region
340
is used to forward-bias the gain section
320
, so that the medium is inverted (for gain), and to back-bias the phase modulation section
310
, so that the index of refraction within the laser cavity can be controlled, either by means of the electro-optic effect, or by a band-edge or quantum-confined Stark effect. A Bragg grating
350
etched into a substrate
370
provides internal reflection within the structure for feedback. The device shown in
FIG. 3
actually uses forward bias in the phase modulation section in order to get large frequency excursions. However, others have made similar devices that use a reverse bias to control the dielectric constant (e.g. as described by Wolf et al., “Modulatable Laser Diode For High Frequencies,” U.S. Pat. No. 5,333,141, issued Jul. 26, 1994.).
In operation, a laser current is applied at the laser electrode
361
to provide a constant current to the gain section
320
to provide the optical gain required for laser operation. A modulation current is applied at the modulation electrode
363
to control the index of refraction with the phase modulation section
310
for modulating the optical frequency of the laser structure
300
.
The performance of the twinguide laser device
300
is limited
HRL Laboratories LLC
Ladas & Parry
Menefee James
Wong Don
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