Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-02-24
2002-09-24
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S046000, C385S042000, C385S010000, C359S333000, C359S885000
Reexamination Certificate
active
06456760
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a method and an apparatus for processing, measuring or storing high-speed optical signals.
2. Description of the Prior Art
An example of prior art optical signal processing circuit is shown in FIG.
1
. In the Figure, the reference numerals
201
and
205
indicate diffraction gratings,
202
and
204
are lenses, and
203
is a spatial filter or an optical storage medium. When a time series signal light is applied to his optical circuit, a Fourier transformation of the time series signal light, that is, a frequency spectrum distribution thereof is formed on the spatial filter
203
by the frequency decomposition function of the diffraction grating
201
and the Fourier transformation function of the lens
202
. When the frequency spectrum distribution is modulated by means of the spatial filter
203
, the waveform of the time series signal can be modulated. Here the wave form can be controlled by the spatial filter
203
even when the time series signal is very high in speed.
As an example, when an optical signal of a pulse width of 200 fs and a pulse interval of 5 ps shown in the upper part of
FIG. 2
is applied, the incident optical spectrum has a shape as shown in the upper part of
FIG. 3
, having an optical power distribution shown by the broken line in the middle part of
FIG. 3
on the spatial filter
203
after passing through the diffraction grating
201
and the lens
202
, and when this is modulated by the spatial filter
203
, it has a shape as shown in the lower part of
FIG. 3
in the spectrum after passing through the spatial filter
203
. A time-dependent waveform corresponding to the spectrum is the pulse sequence shown in the lower part of FIG.
3
. Thus, optical signal processing can be achieved by modulating the frequency spectrum of optical signal by the spatial filter
203
. That is, various waveform shaping according to the filter is possible.
Further, with
203
shown in
FIG. 1
used as an optical storage medium, by applying time series signal light and reference light simultaneously, an interference fringe of both lights is hologram recorded on the optical storage medium
203
. After the recording, when only the reference light is incident, the signal light is reproduced and output. Such studies are reported, for example, in A. M. Weiner, “Programable shaping of Femtosecond optical pulses by use of 128-Element Liquid Crystal Phase Modulator,” IEEE J Quntun Electronics, Vol . 28 No. 4, pp. 908-920 (1992); A. Weiner et al., “Spectral holography of Shaped femtosecond pulses”, Optics Letters, vol. 17, pp. 224-226 (1992).
With the advance in optical communication technology, pulse widths of optical signals utilized in optical transmission are 100 ps (ex; FA-10G system) in the practical application stage, and those of next-generation very large capacity transmission apparatus are considered to utilize picosecond pulses of 1-10 ps. Optical pulses of femtosecond region are application region of the time being in the research and development and material evaluation of stable light sources, and considered not to be applied in optical communications immediately. That is, basic apparatus and method enabling optical pulse generation, waveform shaping, waveform measurement, waveform recording, correlation processing, and the like are required for constructing next-generation very high capacity systems.
However, the above-described prior art has the following problems. That is, in the modulation or hologram recording, all of the diffraction gratings
201
,
203
, the lenses
202
and
204
, and the spatial filter
203
must be laid out in high precision, are liable to be affected by the external environment, are thus difficult to be modular structured, and are nearly impossible to operate other than in a so-called experimental-environment. Therefore, it is not practicable at present stage.
Still further, when treating a time series signal, signal processing is in principle possible by single dimensional diffraction grating and lens. However, the diffraction grating and the lens have a redundant two-dimensional structure, which requires tedious positioning which is inherently unnecessary.
Yet further, when treating a long pulse sequence of over 10 ps or a pulse of large pulse width, it is required to increase the incident beam diameter which, in turn, requires large-sized diffraction grating or lens, and thus a large sized apparatus.
That is, the prior art structure using the diffraction grating pair and the lens effective for femtosecond pulses requires a very large sized apparatus for picosecond pulses, and is difficult to be packaged in a transmission apparatus of about 30×40×3 cm. Further, it is required to use a connection optical system with the optical fiber, and flexible apparatus design according to the pulses is Impossible.
Heretofore, a semiconductor mode-locked laser is known as picosecond pulse generation means.
FIG. 4
shows the structure of a prior art mode-locked laser for use as a short pulse light source.
In the Figure, the mode-locked laser comprises an optical gain medium
51
, an pumping circuit
52
for forming a population inversion to the optical gain medium
51
, mirrors
53
-
1
and
53
-
2
constituting an optical resonator, an optical modulator
54
placed in the optical resonator, and a clock generator
55
for driving the optical modulator
54
. In this construction, when the clock generator
55
drives the optical modulator
54
at a clock frequency equal to the resonance mode spacing of the optical resonator or an integer multiple thereof, an optical short pulse sequence of a repetition frequency equal to the clock frequency or an integer multiple thereof.
FIG. 5
shows the structure of a multi-wavelength light source for simultaneously oscillating light of a plurality of wavelengths.
In the Figure, the multi-wavelength light source comprises an optical gain medium
61
, an arrayed-waveguide grating
62
, a lens
63
for coupling the optical gain medium
61
with the arrayed-waveguide grating
62
, a high reflection mirror
64
and a low reflection mirror
65
disposed at both end surfaces of the optical gain medium
61
, and a high reflection mirror
66
disposed at the other end of the arrayed-waveguide grating
62
.
The arrayed-waveguide grating
62
comprises an input waveguide
71
, an arrayed waveguide
73
including a plurality of waveguides gradually increasing in length by a waveguide length difference &Dgr;L, a plurality of output waveguides
75
, a slab waveguide
72
for connecting the input waveguide
71
and the arrayed waveguide
73
, and a slab waveguide
74
for connecting the arrayed waveguide
73
and the output waveguide
75
, which are formed on a substrate
70
.
Light incident into the input waveguide
71
spreads by diffraction in the slab waveguide
72
, and incident and distributed in equal phase into individual waveguides of the arrayed waveguide
73
. The light-transmitted in the individual waveguides of the arrayed waveguide
73
and reaching the slab waveguide
74
has a phase difference corresponding to the waveguide length difference &Dgr;L. Since the phase difference varies with the wavelength, when focused on the focal plane by the lens effect of the slab waveguide
74
, the light is focused at different positions by wavelengths. Therefore, light of different wavelengths are taken out from the individual waveguides of the output waveguide
75
.
In the multi-wavelength light source using such an arrayed waveguide grating
62
, an optical resonator is formed between the high refection mirror
64
and the high reflection mirror
66
, and light of a plurality of wavelengths can be simultaneously oscillated by steadily exciting the optical gain medium
61
.
However, the prior art mode-locked laser has the following problems.
(1) The oscillation mode envelope spectrum is largely varied by the operation condition, and it is difficult to set the central wavelength and the pulse width.
(2) Since the amplitude and phase of
Ishii Tetsuyoshi
Kurokawa Takashi
Naganuma Kazunori
Okamoto Katsunari
Takenouchi Hirokazu
Nippon Telegraph and Telephone Corporation
Workman & Nydegger & Seeley
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