WO2002101426A2

WO2002101426A2 - Multi-function semiconductor optical amplifier

Info

Publication number: WO2002101426A2
Application number: PCT/US2002/007359
Authority: WO
Inventors: Tong-Ning Li; James K. Eu
Original assignee: Axon Photonics, Inc.
Priority date: 2001-03-09
Filing date: 2002-03-06
Publication date: 2002-12-19
Also published as: WO2002101426A3; AU2002329171A1

Abstract

A multi-function optical device is presented. The optical device includes a strained multi-quantum well semiconductor optical amplifier (114) coupled to a current source (102). The multi-function optical device can operate as a broad-band optical source by coupling light from the strained multi-quantum well semiconductor optical amplifier (SOA) to an optical port (110) and exciting the SOA. By coupling light from opposing sides of the SOA, the multi-function optical device can be utilized as an optical amplifier, an optical switch, a wavelength selective optical amplifier, and a narrow-band source. One embodiment of the multi-function optical device can be configured to provide multiple functions ( e.g., broad-band light source, tunable narrow-band optical source, optical switch, optical amplifier, and wavelength selective optical amplifier).

Description

A Multi-Function Optical Device

Background

1. Field of the Invention

This invention relates to a broad band light source and, in particular, to a light source for application in fiber optic telecommunications.

2. Discussion of Related Art

Optical communications systems, and therefore the components of optical communications systems, are quickly being developed for high bandwidth optical data transmission. High bandwidth systems are in demand for telecommunications networks, metro-systems, local area networks, and cable television subscriber networks, for example.

The information carrying capacity of an optical fiber can be greatly enhanced using wavelength division multiplexing (WDM). In wavelength division multiplexing, data signals are transmitted over an optical beam. The optical beam includes a plurality of light beams, each one of the plurality of light beams having light of a specified wavelength. A single fiber, therefore, can carry the optical beam with individual data signals on each of the plurality of light beams. In other words, data signals can be transmitted on different wavelengths of light through the optical fiber. Wavelength-division multiplexing can be combined with time-division multiplexing as well as other data transmission schemes (e.g., packet transmission) in order to transmit large volumes of data over the network at very high data transmission rates.

Most nodes on the network have the ability to receive signals directed to that node. When wavelength-division multiplexing is used, nodes typically receive signals from light of a subset of the available wavelengths propagating on the optical fiber. Nodes on the network, therefore, need the ability to "drop" one or more of the plurality of light beams so that data signals carried on those dropped light beams can be processed by node opto-electronics. Additionally, at least one of the nodes on the network has the ability to couple signals onto the available light beams of individual wavelengths propagating on the optical fiber. These nodes, then, need to be able to "add" light of one or more of the different wavelengths to the corresponding one or more of the plurality of light beams. The light source utilized in most of these systems is a broad-band lamp (i.e., a white light source). The output of the lamp needs to be broad enough to encompass all of the wavelengths transmitted over the optical system. It is also important that the power output of the lamp be flat over the range of wavelengths in use. However, control of such a lamp is difficult. Additionally, the lamp itself is typically expensive and requires a large amount of operating power. Further, lamps have a limited lifetime and require frequent replacement.

Additionally, even though the lamp has a high operating power, the bandwidth can be very wide (e.g., greater than 300 nm) and therefore the output power for the useful range of wavelengths can actually be very low. In the range of 20 nm around 1550 nm, for example, the output power may be as low as -45 dBm and outside this range it may reach as low as -75 dBm.

A second traditional broadband light source includes a four edge emitting LEDs (EELED). A broad bandwidth light source can be accomplished only by cascading the four EELEDs. However, this is costly and the output power is still low (e.g., about -37 dBm).

Therefore, there is a need for less expensive and more robust light sources with high output powers in the operable ranges for use in optical fiber networks and for use in testing of optical fiber components.

Summary

In accordance with the present invention, a multi-functional optical device is presented that utilizes the features of a semiconductor optical amplifier (SOA). The multi-functional optical device includes a semiconductor optical amplifier, a current source, and at least one optical coupler. In some embodiments, the optical coupler couples light from the SOA to an optical fiber.

In one embodiment, the multi-functional optical device operates as a broadband light source. The SOA will output broad-band light of good intensity when excited by the current source. Light can then be coupled from the SOA into an optical fiber. In one embodiment, two output optical fibers are provided, one for each of two opposing sides of the SOA. The two light sources provided, then, can be of equal intensity.

In an embodiment with two optical fibers coupled to the SOA, the multifunctional optical device can operate as an optical switch or an optical amplifier as well as a two-output optical source. As a programmable gain amplifier, light is input through one optical fiber and coupled into the SOA. The SOA amplifies the light relative to a user-defined input to the current source. The amplified light is output through a second optical fiber. As an optical switch, light is input through one optical fiber and passes through the SOA if the current source is set high enough. However, if the current source is off, the light input is absorbed in the SOA.

In one embodiment, the multi-functional optical device includes a tunable optical filter coupled in series with the SOA. Therefore, as a light source the multifunctional optical device can be tuned to output specific wavelengths of light. As an optical amplifier, the multi-functional optical device is wavelength selective and can be used, for example, to evaluate optical components with large insertion losses.

Embodiments of the present invention can provide a constant, broad band, dual light source which can be utilized for testing and monitoring of filtering characteristics during production of filtering components (thin film filters or arrayed wave guide filters) and for testing and monitoring of other optical components such as fibers, connectors, circulators, add/drop components or any other optical component.

Additionally, embodiments of the present invention can provide dual light sources for continuous operation of optical networks.

These and other embodiments are further discussed below along with the following figures.

Brief Description of the Figures

Figure 1 shows a block diagram of a multi-function optical device according to an embodiment of the present invention.

Figure 2 shows a block diagram of a Semiconductor Optical Amplifier for use in a broad-band light source according to the present invention.

Figure 3 shows the amplifier bandwidth for an SOA for use in a broad-band light source according to the present invention. Figure 4 shows a multi-function optical device according to the present invention.

Figure 5 shows a controller circuit for a multi-function optical device according to the present invention. In the figures, components that are identically labeled have the same or similar functions.

Detailed Description

Figure 1 shows a block diagram of a broad-band light source 100 according to an embodiment of the present invention. Broad band light source 100 includes a semiconductor optical amplifier (SOA) 101 coupled to a current source 102. SOA 101 is capable of outputting light when properly excited by current source 102. Further, the light from SOA 101 can be coupled into optical fiber 104 by, for example, a collimator 103. Collimator 103 may further include focusing lenses and other optical components to better facilitate the coupling of light into optical fiber 104. Additionally, light from SOA 101 may further be coupled into optical fiber 106. Collimator 105 couples light from SOA 101 into optical fiber 106. Further, collimator 105 may include optical components in order to facilitate the coupling of light into optical fiber 106.

Current source 102 may be an externally controlled variable current source. If current source 102 is turned low, or off, then SOA 101 will not output light. Further, any light input on optical fiber 106 or optical fiber 104, which may be coupled into SOA 101, will be absorbed. After a particular threshold, SOA 101 will output light in response to input light on optical fiber 106 with a gain dependent on the output of current source 102. Multi-function optical device 100, then, may function as an optical switch or a variable-gain amplifier. If the output of current source 102 is high enough, and there is sufficient reflection from output surfaces 110 and 111 of SOA 101, then SOA 101 will output light regardless of any input light. The light output from SOA 101 under these conditions is broad-band (i.e., with a 3dB width of 70 nm around about 1550 nm, for example) and is therefore ideal for use in wavelength division multiplexing systems.

In order to stabilize the operation of SOA 101, some embodiments of multifunction optical device 100 include a temperature control circuit 114. Temperature control circuit 114 holds the temperature of SOA 101 at a predetermined temperature in order to optimize the operation of SOA 101. In one embodiment, the temperature of SOA is held at about 20° C. In some embodiments, the temperature is held to about ±0.1° C.

In some embodiments, a multi-functional optical device 100 includes one or more tunable filters (tunable filters 112 and 113 are shown in Figure 1). Tunable filters 1 12 and 113 allow multi-functional optical device 100 to provide light with tunable wavelengths on one or both of optical fibers 104 and 106. Further, tunable filters 112 and 113 allow multi-functional optical device 100 to operate as a wavelength selective amplifier. Each of tunable filter 112, collimator 105, and SOA 101 may be optically coupled by optical fiber. Additionally, each of tunable filter 113, collimator 103 and SOA 101 may be optically coupled by optical fiber.

Figure 2 shows an example of a semiconductor optical amplifier 150. Optical amplifier 150 includes a substrate 151 and an active region 152. In most cases, the active region is Inι_-xGa_xAsι_-yP_y based technology. In typical operation, optical amplifier 150 is coupled to an input optical fiber 153 and an output optical fiber 154.

Optical amplifier 150 is coupled to a voltage source V_g in active region 152 and a ground at substrate 151. If unpowered, active region 152 absorbs incident light. When a voltage is supplied at V_g, then charge carriers in active region 152 are promoted to higher energy levels. Photons incident on active region 152, instead of being absorbed, initiate transitions from the higher energy levels into lower levels and the production of further photons, as in a laser device. Therefore, the amount of amplification (i.e., the number of photons emitted by active region 152 for each photon incident on active region 152) is dependent on the number of electrons held in the excited state in active region 152, which depends on the current and voltage supplied at V_g.

If light is incident on active region 152 from optical fiber 153, light is emitted from active region 152 and coupled into optical fiber 154. The gain of amplifier 101 is determined by an input signal at V_g. In many instances, optical amplifier 101 has an "on" state when V_g is above a threshold level and an "off state when V_g is below the threshold level.

In some cases, a current source 102 is provided to add current to amplifier 101, turning SOA 101 "on" or "off." For example, in a typical SOA a current of 40 mA may turn the SOA on with a gain of 5 dB at 1550 nm and 2.5 dB at 1530 nm. The SOA may saturate at 100 mA, providing a gain of 0 dB at 1530 nm and 5 dB at 1550 nm.

Semiconductor optical amplifiers (SOAs) have several advantageous properties. SOAs are fast switching (on the order of about 0.5 to about 1 ns), making them very attractive for dynamic, programmable optical systems requiring fast switching times. Additionally, SOAs are easily integrated with other photonic devices in a planar waveguide.

The substitute of conventional bulk InGaAsP gain regions with multiple quantum well (MQW) technologies yields further advantages for SOAs. For example, the saturation output power is enhanced, allowing for high gains. MQW-SOAs typically have a broad-band gain spectrum, allowing for amplification over a broad range of wavelengths (about 70 to 80 nm in some cases). MQW-SOAs further yield low noise figures, approaching the quantum limit. Polarization sensitivity of the MQW-SOA can be overcome successfully by employing both tensile and compressively strained quantum wells in a single active layer. Further advantages for utilizing multiple quantum well semiconductor optical amplifiers include their reliability, linearity of their amplification with signal input power, and reliable threshold current densities.

One skilled in the art is familiar with the use and operation of such semiconductor optical amplifiers. A further discussion of such optical amplifiers and the operation of such optical amplifiers is given in This, et al., "Progress in Long- Wavelength Strained-Layer InGaAs(P) Quantum- Well Semiconductor Lasers and Amplifiers," IEEE J. Quantum Elect., Vol. 30, No. 2, p. 477-499 (Feb., 1994), herein incorporated by reference in its entirety. In a typical SOA, the gain of the amplifier depends on the polarization of the input light beam. The gain can differ by as much as 5 to 8 dB for transverse-electric (TE) and transverse-magnetic (TM) polarisations. Transverse-electric and transverse magnetic polarisations refer to electric field directions with the magnetic field perpendicular to the boundary between the substrate and the active region and parallel with the boundary between the substrate and the active region, respectively. In general, the gain g of SOA 101 can be expressed as

g = (T _g /V)(N- N₀) , (1) where T is the confinement factor, σ_g is the differential gain coefficient, V is the active volume, and No is the value of N required at transparency. The confinement factor T is related to the amount of light confined within the active region of the SOA. The differential gain coefficient σ_g refers to gain cross active region of SOA. Finally, N refers to the number of electrons in the excited state and therefore is related to the voltage and current being supplied to pump the SOA.

Since both T and σ_g are dependent on the particular polarization, the gain of a typical SOA is dependent on the polarization state of the input beam. This is an undesirable property of the SOA since the polarization state of the beam often changes during propagation. This is also an undesirable property for light sources since the output light beam would then have a strong polarization dependence.

In some embodiments, semiconductor optical fiber 101 is a strained multiple quantum well semiconductor optical amplifier 101. With the advancement of MOCVD processing technologies, SOAs can now be produced with strained multiple quantum well (MQW) technologies. A conventional SOA has a larger transverse electric (TE) mode gain than transverse magnetic (TM) mode gain because the light emission occurs between the conduction band and the heavy hole valence band. Emission of that time yields a TE gain larger than the TM gain. In a strained multiple quantum well semiconductor optical amplifier, polarization insensitivity can be realized. A tensile strain shifts the emission to the conduction band and a light-hole valence band, which increasesTM mode gain relative tot he TE mode gain. The tensile strain can be controlled in the MOCVD process utilizied to grow active region 152 of SOA 101. The tensile strained barrier MQW structure can be grown by adjusting the composition (i.e., the values of x and y) of the active material In ι_-x Ga_x As_1-y P_y material. The particular stoichiometry of the material of active region 152 can create a tensile strain in order to enhance the high TM mode gain. In the growth process, then, both parameters T and σ_g can be controlled in order to increase the design allowance for polarisation-insensitive optical amplifiers. For example, growth of a composition of InGaAsP with x being about 0.37 and y being about 0.0 yeilds a strained multiple quantum well semiconductor optical amplifier 101 with substantially no polarization dependent gain.

In strained MQW SOA 101, the gain coefficient is given by ^{g{ω) =} ₊ {ω- _ϋf_T (2)

where go is the peak value of the gain determined by the pumping level of the amplifier, ω is the optical frequency of the incident signal, ωo is the atomic transition frequency, and T₂ is the dipole relaxation time, which is typically quite small (about 0.1 ps to about 1 ns). The pumping level of SOA 101 is determined by V_g and the current supplied by current source 102. As shown in Equation 2, the gain is maximum when the incident frequency ω coincides with the atomic transition frequency ω₀. As can be further seen from Equation 2, the gain g(w) is independent of polarization. The gain reduction for ω≠ω₀ is governed by a Lorentzian profile that is a characteristic of homogeneously broadened two-level systems.

In practice, however, the gain spectrum of strained MQW SOA 101 can vary significantly from the theoretical Lorentzian profile. The gain bandwidth is defined as the full width at half maximum (FWHM) of the gain spectrum g(ω). For the Lorentzian spectrum the gain bandwidth is given by

Δ«„

Aυ = r, (3)

2π πT,

The gain bandwidth, then, is about 3 THz forT₂ about 0.1 ps. Amplifiers with a relatively large gain bandwidth are desired for optical communications systems since it is desirable to have the actual gain nearly a constant over the entire bandwidth of a multichannel signal. The amplification factor can be determined with the standard Fabrey-Perot interferometer calculations and is given by

(4)

where P and R₂ are the reflectivity of faces 155 and 156 of SOA 101, v_m represents the resonance frequencies of the cavity created between faces 155 and 156, Δv_L is the longitudinal-mode spacing, also known as the free-spectral range of the Fabrey-Perot cavity created between faces 155 and 156, and G(v) is the single-pass amplification factor. The single-pass amplification factor G(v) corresponding to that of a transverse-wave (TW) SOA amplifier, such as SOA 101, can be expressed as

G(ω) = e^g(ω)l . (5)

The amplifier bandwidth Δv_A is defined as the full width at half maximum of G(ω) and is related to the gain bandwidth Δv_g by the equation

(6)

As can be seen from Equation 6, the amplifier bandwidth is less than the gain bandwidth, the difference depending on the amplifier gain itself. The frequency dependence results mainly from the frequency dependence of G(v) when G(V)

is less than about 0.17. The atomic transition frequency is very broad and therefore the gain width is also broad relative to the frequency.

In accordance with the present invention, SOAs have been developed with 3dB amplifier bandwidth, which is a bandwidth of about 70 nm (9THz). Figure 3 shows a gain spectrum of an SOA according to the present invention with 100 mA of driving current. This bandwidth reflects the relatively broad gain spectrum g(ω) of SOAs in general. In particular, Figure 3 shows a relatively flat output of an SOA such as SOA 101 (Figure 2) over the range from about 1530 nm to about 1610 nm. As shown in Figure 1, SOA 101, which can be a strained multi-quantum well semiconductor optical amplifier as discussed above, provides high output power and a broad bandwidth. From a single SOA, two output ports, each capable of operating as a broad band light source, can be obtained. In one embodiment, the power output on each port is greater than -8dBm over a width of about 70 nm around a wavelength of about 1550 nm (1550 nm ± 20 nm). In addition, the output power stability is less than 0.02dB over a period of 8 hours of operation. Table 1 shows the operating characteristics of one embodiment of a dual output broadband light source according to the present invention. Table 1

For comparison, a traditional white light source has a very broad bandwidth (greater than about 300 nm). Although the total output power may be high, the output power in useful ranges is typically very low. Typically, an output power of greater than -45 dBm can be realized only in a 20 nm region around the center frequency. A light source according to the present invention realizes an output power of greater than -8 dBm in a region of about 70 nm around the center wavelength of 1550 nm, which is a high power in the useful range for WDM applications.

Alternatively, cascaded edge-emitting LEDs (EELEDs) can also be utilized as a broad band light source. With four (4) cascaded EELEDs, the output power is still low (about -37 dBm in the applicable range) and the cost of the light source is significantly increased. Multi-functional optical device 100 can utilize both the high carrier density performance of strained MQW SOA 101 and also the traveling wave performance. Figure 4 shows an example configuration of a multi-functional optical device 100. Multi-functional optical device 100 includes a housing 400 with optical ports 401 and 402. Light can be coupled from optical fibers into or out of optical ports 401 or 402. Optical device 100 may be equipped with a power toggle switch 403. An optical configuration such as that shown in Figure 1 is mounted within housing 400 so that optical fiber 106 is coupled to optical port 401 and optical fiber 104 is coupled to optical port 402. Adjustable input 404 controls the current and voltage to SOA 101. Therefore, the gain of SOA 101 is controlled by an external user. In some embodiments, adjustable input 404 is continuously adjustable so that the user can adjust the current and voltage to SOA 101. In other embodiments, adjustable input 404 can be switched between an off position and a position of fixed gain.

Some embodiments may have tunable filters (e.g., tunable filter 112 or 113 of Figure 1). In those embodiments, adjustable input 405 controls tunable filter 112 and adjustable input 406 controls tunable filter 113. In one embodiment, tunable filters 112 and 113 may be set to pass all wavelengths or may be set to pass only particular wavelengths of light.

Multi-functional optical device 100 can be utilized, for example, as a broadband light source, as a single wavelength light source, as an optical amplifier, as a wavelength selectable amplifier, or as an optical switch. As a broad-band light source, tunable filters 112 and 113 are set to pass all wavelengths and adjustable input 404 controls the intensity of the light output at ports 401 and 402. As a single wavelength light source, tunable filters 112 and 113 are set to pass the desired wavelengths at ports 401 and 402, respectively, and adjustable input 404 is adjusted to control the intensity of the light at ports 401 and 402. As a broad-band optical amplifier, tunable filters 112 and 113 are set to pass broad-band light, light is input on either of ports 401 or 402, and adjustable input 404 controls the gain of SOA 101 by controlling the current and voltage to SOA 101. As a wavelength selectable amplifier, one or both of adjustable inputs 405 and 406 are adjusted to the selected wavelength, light is input on either of ports 401 or 402, and adjustable input 404 is adjusted to control the gain of SOA 101. As a switch, adjustable input 404 is adjusted between below a threshold value where all light is absorbed by SOA 101 and a value to provide a fixed gain.

Figure 5 shows an embodiment of the controlling circuit that is mounted within housing 400 of Figure 4. Controlling circuit 500 includes power supply 501, temperature controller 502, and current source 503. Current source 503 is coupled to SOA 101 to control the gain of SOA 101. The circuit consists of 4 parts, power supply for ± 5 volt, power feed back controller, temperature control and LCD display. Combine power control and temperature control the output power stability of 0.05 dB can be achieved for 8 hours. The discussion of embodiments of the invention is intended to be exemplary only. One skilled in the art will recognize various modifications to the examples which are tended to be within the scope of this disclosure. Therefore, the invention is limited only by the following claims.

Claims

ClaimsWe claim:

1. A multi-functional optical device, comprising: a strained multi-quantum well semiconductor optical amplifier; an optical port coupled to the strained multi-quantum well semiconductor optical amplifier; and a control circuit coupled to control the gain of the strained multi- quantum well semiconductor optical amplifier.

2. The device Claim 1, further including a second optical port coupled to the strained multi-quantum well semiconductor amplifier.

3. The device of Claim 1 , further including a tunable filter coupled between the strained multi-quantum well semiconductor optical amplifier and the optical port.

4. The device of Claim 2, further including a second tunable filter coupled between the strained multi-quantum well semiconductor optical amplifier and the second optical port.

5. The device of Claim 1, further including a user adjustable current source, the user adjustable current source determining the gain of the strained multi-quantum well semiconductor optical amplifier.

6. A method of providing a broad-band light source, comprising: providing a strained multi-quantum well semiconductor optical amplifier; providing current to the strained multi-quantum well semiconductor optical amplifier; coupling light from the strained multi-quantum well semiconductor optical amplifier to an optical port.