Optical waveguides – Directional optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2001-06-29
2003-07-29
Epps, Georgia (Department: 2873)
Optical waveguides
Directional optical modulation within an optical waveguide
Electro-optic
C385S010000, C385S008000, C359S315000
Reexamination Certificate
active
06600844
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of optical communications and, more specifically, to the modulation of optical pulse streams. In particular, the modulation of the pulse streams is obtained by applying controlled delays to the optical pulses in the pulse stream.
BACKGROUND OF THE INVENTION
Many satellite and terrestrial optical communication systems require transmission of analog optical signals. The straightforward way to transmit an analog optical signal is to modulate the amplitude of an optical carrier. This approach, however, suffers from poor signal-to noise ratio (SNR). It is well known that broadband modulation techniques, which utilize higher bandwidth than that of the transmitted waveform, may improve the SNR over that achieved with amplitude modulation. Pulse Position Modulation (PPM) is one of these techniques. In PPM, an optical pulse stream samples an analog signal. A temporal shift in the pulse position of each optical pulse represents a sample of the transmitted waveform. Thus, the temporal position of each pulse is shifted from its unmodulated position in proportion to the amplitude of the analog signal. The improvement in SNR near the Nyquist sampling frequency of a pulse position modulated signal over an amplitude-modulated signal is shown below:
SNR
PPM
SNR
AM
(
t
p
/&tgr;)
2
Eq.1
where t
p
is the temporal spacing between unmodulated pulses and &tgr; is the pulse duration of each pulse, respectively.
Therefore, the optical pulses used for PPM should be of short duration since SNR performance improves as the pulse widths within the modulated pulse stream decrease. Pulse widths as short as 0.3 picoseconds may be desirable for a PPM optical communication system. However, it is also well known in the art that PPM performance will suffer if the shapes of the optical pulses vary or the amplitudes of the pulses vary on a pulse-to-pulse basis. Mode locking of a pulsed laser is a mature technique for producing equally spaced ultra-short identical pulses. It would be beneficial to use a mode locked laser in a PPM communication system if the equally spaced pulses produced by the system could be modulated without distortion.
Furthermore, the PPM system should be capable of supporting the modulation and transmission of analog signals with large bandwidths. Typically, a bandwidth of &Dgr;f=1-10 GHz and higher is of interest for inter-satellite communications. Since pulse repetition frequencies (PRF) of 1/t
p
>2&Dgr;f are required for sampling a signal with a bandwidth of &Dgr;f, trains of picosecond pulses with a PRF over one gigahertz should be used for realizing the advantages of PPM. For example, an optical inter-satellite link designed to transmit waveforms with a bandwidth &Dgr;f=20 GHz requires a sampling rate with a PRF=1/t
p
≧2&Dgr;f=40 GHz. At a sampling rate of 40 GHz and optical pulses with 1 picosecond duration, a 30 dB gain is realized over an AM system with equal optical power.
Implementations of PPM for optical communications require a mechanism for modulating the delays between extremely short optical pulses within a pulse stream without modulating the shapes or pulse-to-pulse amplitudes of the pulses. Direct modulation of a semiconductor laser will appropriately modulate the delay between the optical pulses generated by the laser. However, a directly modulated semiconductor laser generates relatively long pulses that result in limited SNR performance. Pulse compression can be used on the longer pulses produced by the directly modulated semiconductor laser, but devices to provide such compression are complex and cumbersome. Direct modulation of a semiconductor laser may also introduce amplitude modulation or pulse reshaping of the individual time-shifted pulses, further limiting performance.
Pulse position modulation of extremely short optical pulses is also achieved by applying a pulse-to-pulse delay external to the source of the equally spaced optical pulses. That is, a modulator is used that can receive a stream of optical pulses, change the pulse-to-pulse delay at the rate required for properly sampling the transmitted analog signal, and further transmit the delayed pulses. It is known in the art that one example of a pulse position modulator for optical pulses consists of an optical delay line, such as a parallel slab of transparent electro-optically active material. The refractive index of the electro-optically active material can be controllably varied by an applied voltage, so that each pulse is controllably delayed upon traversing the electro-optically active material in accordance with the instantaneous voltage. However, such a modulator requires an undesirably large amount of electrical power, due to the relatively large voltages required to modulate the refractive index of the material and thus modulate the delay encountered by a pulse traversing the material.
Another example of a pulse position optical modulator relying upon the use of electro-optically active material is disclosed in U.S. Pat. No. 3,961,841, issued Jun. 8, 1976 to Giordmaine. Giordmaine discloses a device for optical pulse position modulation comprising a diffraction grating in combination with an electro-optic prism and a lens. The diffraction grating splits an incident light pulse into its frequency components and the lens directs the components into the prism. The refractive index change provided by the prism causes a phase shift in the frequency components and thus a time shift in the optical pulse once it is reconstructed by the diffraction grating. The device disclosed by Giordmaine provides the capability of modulating light pulses as short as one picosecond. However, the maximum controllable delay is limited to a few picoseconds for a 3 picosecond pulse and further decreases for shorter pulses. Also, the multiplicity of optical elements such as the diffraction grating, lens, and prism increase the complexity and manufacturing cost of the device.
A device for delaying optical pulses is disclosed in U. S. Pat. No. 5,751,466, issued May 12, 1998 to Dowling et al. and is shown in FIG.
1
. Dowling discloses a photonic bandgap structure comprising a plurality of cells 18A-18N of width d in which the refractive index varies. The refractive index variation may be such that each cell comprises two layers of materials with two different indices of refraction n
1
and n
2
. If the widths of the two layers within each cell are &lgr;/4n
1
and &lgr;/4n
2
where &lgr; is the free space wavelength of the optical pulse to be delayed, a distributed Bragg reflector structure is created. According to Dowling, the thickness and/or number of layers in the photonic bandgap structure and/or their indices of refraction are selected to produce a structure with a transmission resonance center frequency and bandwidth corresponding to the frequency and bandwidth of the optical pulse to be delayed. By matching the transmission resonance to the optical pulse, a controllable delay is imparted to the optical pulse without significantly altering the optical signal.
The device disclosed by Dowling requires that the thickness of each layer in the device be approximately one-half the wavelength of the incident optical pulse to form the photonic bandgap structure. The delay imparted on an optical signal by transmission through the structure will depend upon the number of layers and the indices of refraction within the layers. The structure can be thought of as essentially increasing the length of the waveguide in which it is contained, thus providing the desired delay. For example, Dowling discloses a simulation of a photonic bandgap structure that is 7 &mgr;m thick that provides a delay equivalent to an optical signal traveling through a 110 &mgr;m structure, or a delay of about 0.4 picoseconds. Since the amount of delay from a single structure is relatively small, Dowling discloses that the structures can be successively coupled in a single device to provide additional delay. Of course, this increases the overall size of
Epps Georgia
Hindi Omar
HRL Laboratories LLC
Ladas & Parry
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