Fiber tail assembly with optical signal tap

Details Fiber tail assembly with optical signal tap Fiber tail assembly with optical signal tap

2002-11-27

2004-09-21

Tremblay, Mark (Department: 2876)

Optical waveguides

With optical coupler

Particular coupling structure

Reexamination Certificate

active

06795620

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to monitoring the operation of an optical modulator, and more particularly to methods and devices for tapping light from radiation modes of the optical modulator for monitoring the optical power in a guided mode without tapping light from the guided mode. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for tapping monitor light for controlling the bias of an optical modulator.
An integrated optical modulator is of great importance for operating a fiber optical communication system, especially for operating in the range of 2.5 to 40 Gbps (gigabits per second). An optical data signal may be generated by directly modulating an optical source, such as a semiconductor laser, through modulation of the laser's electrical drive-current. However, high-speed direct modulation can induce wavelength fluctuation (chirp) in the optical signal, which can lead to wavelength dispersion in an optical fiber and degrade signal transmission.
Alternatively, an optical signal with significantly reduced chirp or with well controlled chirp can be generated by using a continuous-wave optical source that is externally modulated. For example, an external modulator may be a Mach-Zehnder type modulator
100
, as shown in FIG.
1
. Modulator
100
has an input waveguide
104
, a splitting branch
108
, two modulation waveguides
114
and
116
, a recombination branch
120
, and an output waveguide
124
, all on a substrate
102
. The waveguides may be formed in substrate
102
in any way, such as by selectively diffusing a metal, such as titanium (Ti), into the substrate to form a waveguide that has a higher refractive index than the surrounding material. In this case, the surrounding material acts as a lower-index medium, and light is guided to propagate along the higher-index waveguide. Alternatively, additional material layers may be deposited onto substrate
102
to act as cladding material or waveguide material.
In this example, a light source, such as a semiconductor laser diode that is not shown, provides continuous-wave light to input waveguide
104
. The source light may be distributed by splitting branch
108
into two separate light fields that propagate through modulation waveguides
114
and
116
respectively, where each is individually phase modulated. The light fields are added by recombination branch
120
into output waveguide
124
, and the amount of light that enters output waveguide
124
depends on the optical phase difference between the light fields from modulation waveguides
114
and
116
.
More specifically, if the light fields are in phase, with zero phase difference, then all of the light may recombine into a propagating guided mode
150
that travels along output waveguide
124
. Alternatively, if the light fields are out of phase, with 180 degree phase difference, then all of the light may recombine into a primary radiation mode
152
. As shown in
FIG. 1
, primary radiation mode
152
may be anti-symmetric, with a field profile that has two lobes that are 180 degrees out of phase from each other. Primary radiation mode
152
is not guided by output waveguide
124
and may travel and diffractively spread in substrate
102
. As a further alternative, if the phase difference is an intermediate value, then all of the light may be distributed between guided mode
150
and primary radiation mode
152
. When the phase difference changes, the optical powers in guided mode
150
and primary radiation mode
152
vary accordingly, in a manner complementary to each other.
The phase difference depends on the optical phase of the light from each modulation waveguide
114
and
116
, which in turn depends on the refractive index of each waveguide's material. For example, the material of substrate
102
may be lithium niobate (LiNbO
3
) that has an electrooptic effect, and the refractive index of waveguides
114
and
116
may be electrically modulated. Modulation may be done through any type of electrode, such as a travelling-wave electrode that accommodates broadband modulation signals.
As described above, modulating the phase difference results in modulating the optical power in guided mode
150
. The phase difference may be modulated about a bias point, which depends on modulation voltages applied, the wavelength of the source light, and the temperature and mechanical stress of the modulator. The bias point may drift over time, degrading the extinction ratio of modulator
100
. However, the bias may be controlled through monitoring the modulator output power.
In an ideal modulator, the output power may be monitored by monitoring any portion of primary radiation mode
152
. This is because light traveling through the modulator experiences no optical loss or scattering, and the optical power in guided mode
150
and that in primary radiation mode
152
are complementary. The modulation-responses of the two modes are in counter-phase, as shown in FIG.
2
. Modulation quadrature points Q, of guided mode
150
, and q, of primary radiation mode
152
, occur at the same quadrature voltages. Furthermore, any portion of primary radiation mode
152
may be sampled, and the sample signal is complementary to the output power, with any necessary amplitude scaling applied.
However, in a practical modulator, complementarity of any one portion of primary radiation mode
152
may be destroyed. A variety of secondary radiation modes may be produced that combine with the primary radiation mode
152
to form a combined radiation field
158
. Combined radiation field
158
and guided mode
150
may be different in amplitude, phase, and quadrature voltages, as shown in FIG.
3
. Furthermore, the differences may vary with time. Therefore, merely monitoring part of combined radiation field
158
may not be sufficient to monitor the power of guided mode
150
.
Secondary radiation modes may be caused by partial scattering of light traveling in the waveguides. Such scattering may include propagation scattering, splitting/bending scattering, and coupling scattering. Propagation scattering may be caused by the roughness of a waveguide/cladding boundary. For example, part of the light that propagates through waveguides
104
,
114
,
116
or
124
may be continuously scattered into substrate
102
. Splitting/bending scattering occurs wherever there is a bend in a waveguide or a change in waveguide cross-section, such as at splitting branch
108
or at each bend of waveguides
114
and
116
. Coupling scattering occurs when light is coupled from one waveguide into another, such as occurs in coupling from an output waveguide into an output optical fiber.
Some secondary radiation modes are modulated and some are not. For example, a secondary radiation mode generated at splitting branch
108
is not modulated, because the scattering occurs before the modulation waveguides
114
and
116
. In contrast, light scattered subsequent to the modulation waveguides may be modulated. The radiation modes are not confined to the waveguides and propagate through the substrate. At the output face of modulator
100
, the secondary radiation modes spatially overlap with primary radiation mode
152
, and all modes add to create the combined radiation field
158
. However within the combined field, only the two-lobed field of primary radiation mode
152
is complementary to guided mode
150
. Optical interference among the primary and secondary radiation modes can significantly distort combined radiation field
158
and destroy its complementarity to guided mode
150
.
As an example, combined radiation field
158
may be sampled by a photodetector across one lobe of the field profile and compared with guided mode
150
. The power of guided mode
150
is proportional to cos
2
(&pgr;V/2V
&pgr;
), where V and V
&pgr;
are an applied modulating signal and the half-wave voltage of the modulator respectively. The same dependence governs amplitude-modulated secondary radiation modes, such as those generated in the bends o

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