Parameters obtained from the polarization dependence of the photocurrents of GaAs and Si PDs at a wavelength of 1.55 μm.
Incident light with a photon energy induces two-photon absorption (TPA) when, where is the band gap of the photo-absorption layer of a photodiode (PD). Because the absorption coefficient is small, photocurrent generated by TPA is too low to be used in conventional optical signal receivers. However, the nonlinear dependence of the photocurrent on the incident light intensity can be used for optical measurements and optical signal processing. It has been used for autocorrelation in pulse shape measurements , dispersion measurements [2,3] and optical clock recovery . These applications exploit the dependence of the generated photocurrent on the square of the instantaneous optical intensity. Measurement systems using TPA in a PD can detect rapidly varying optical phenomena without using high speed electronics.
This chapter reviews research on TPA and its applications at the optical fiber transmissionwavelength. Theory of TPA for semiconductors with diamond and zinc-blende crystal structures is reviewed. In contrast to linear absorption for which the photon energy exceeds the band gap, the TPA coefficient depends on the incident lightpolarization. The polarization dependence is described by the nonlinear susceptibility tensor elements.
The polarization dependences of TPA induced by a single optical beam in GaAs- and Si-PDs are compared to evaluate the effect of crystal symmetry. It is found that, in contrast to the GaAs-PD, TPA in the Si-PD is isotropic for linearly polarized light at a wavelength of 1.55 μm. Photocurrents for circularly and elliptically polarized light are also measured. Ratios of the nonlinear susceptibility tensor elements are deduced from these measurements. The different isotropic properties of GaAs- and Si-PDs are discussed in terms of the crystal and band structures.
Cross-TPA between two optical beams is also studied. The absorption coefficient of cross-TPA strongly depends on the polarizations of the two optical beams. It is shown that the polarization dependence of cross-TPA is consistent with the nonlinear susceptibility tensor elements obtained from the self-TPA analysis.
Cross-TPA can be applied to polarization measurements. Photocurrents generated in the Si-PD by cross-TPA between asignal light under test and a reference light are used to detect the polarization. The light under test is arbitrarily polarized and its Jones vector can be determined by photocurrents generated by cross-TPA. This measurement method can detect the instantaneous polarization when the reference light temporally overlaps with the light under test. Because the time division is limited only by the pulse width of the reference light, it is possible to detect rapid variationsin the polarization. This method can measure not only the linear polarization direction but also the elliptical polarization. Applications to measurement of the output optical pulse from an optical fiber with birefringence and a semiconductor optical amplifier are demonstrated.
2. TPA in semiconductors with diamond and zinc-blende crystals
2.1. Polarization dependence
TPA is a third-order nonlinear optical process. Third order nonlinear polarization is induced by the optical electric field according to
Although the third-order nonlinear susceptibility tensor contains 34 elements, the number of non-zero independent elements is limited by the crystal symmetry and the properties of the incident light. It is apparent that relations
We consider cross- and self-TPA between two optical beams. The electric field is the sum of the electric fields of thetwo incident optical beams.
The first and second terms are polarization induced by the self- and cross-electric field effects, respectively. Terms proportional to the inner product of and are invariant for rotation of axes and are isotropic. In contrast, terms that are proportional to
Two optical beams propagate in the crystal under the effect of self- and cross-TPA.
2.2. Estimate of photocurrent induced by TPA in PDs
Commercially available PDs are usually designed to be used for photon energies greater than the band gap of the photoabsorption layer. As the absorption coefficient is about 105 cm-1, absorption layer is several micrometers thick. On the other hand, the absorption coefficient is much smaller for TPA. If we consider only self-TPA, Eq. (6) is solved as
When optical pulses with an intensity density
3. Experimental setup
Because the photocurrent of PD is proportional to the square of the instantaneous light power density, it is necessary to concentrate the optical power into a narrow spatial region and a short time period. Thus, a short pulsed light beam is more suitable for TPAmeasurementsthan continuous wave light.
Figure 1 shows the experimental setup. A gain-switched laser diode (LD) generated optical pulses with a wavelength of 1.55 μm, a pulse width of 50 ps and a repetition rate of 100 MHz. Light pulse from the gain-switched LD exhibit large wavelength chirping. The pulse was compressed to about 10 ps by an optical fiber with positive wavelength dispersion. Its peak power was then amplified using an Er-doped fiber amplifier (EDFA) to further compress the pulse width through the nonlinear soliton effect in a normal-dispersion fiber. The final pulse width was compressed toabout 1 ps.
To measure cross-TPA between two optical beams, a second gain switched LD with a wavelength of 1.55 μm was prepared. Noise due to interference between the two beams does not affect the measurement because the optical frequency difference between the two beams is greater than the bandwidth of the measurement system. Pulse with a repetition rate of 100MHz are completely synchronized with those of the first optical beam. The second optical beam is also amplified by an EDFA.
Both the two beams were made linearly polarized by polarization controllers. After they were launched into free space, they passed through polarizing beam splitters to ensure that they were completely linearly polarized. Half-wave or quarter-wave plates were inserted if it is necessary to control the polarization of the beams. The two beams were spatially overlapped by a polarization-independent beam splitter and they were focused on a PD. It was confirmed that the polarization did not change on reflection at the polarization-independent beam splitter by monitoring the polarization before and after reflection. An optical power meter was placed at the location of the PD and it was used to check if the optical power was independent of the polarization.
When two optical beams are illuminated on a PD, photocurrents due to self-TPA and cross-TPAare simultaneously generated. It is necessary to detect only the photocurrent generated by the cross-TPA. Optical pulse streams were mechanically chopped at frequencies of 1.0 and 1.4 kHz. Electrical pulsesthat had been synchronized with mechanical choppers were fed into a mixer circuit that generated a sumfrequency of 2.4 kHz. These generated electrical pulses with the sum frequency were used as the reference signal for the lock-in amplifier. Thus, the lock-in amplifier detected only the photocurrent generated by two-beam absorption, that is, cross TPA.
4. Pulse width measurement by cross-TPA
Cross-TPA was used to measure the pulse width generated by the pulse compression process described in the previous section. After the compressed optical pulse was divided into two branches by an optical fiber beam splitter, the timing between them was controlled by a variable delay line. They were then irradiated on the Si-PD. The two beams were made orthogonally linearly polarized to suppress noise due to interference. The photocurrent generated by cross-TPA between the divided two optical beams is
Figure 2 shows the self-correlation trace of the compressed optical pulse. The photocurrent due to the cross-TPA is generated only when the two optical pulses temporally overlap on the PD. It disappears when the time delay is larger than the pulse width. The self-correlation trace has a full-width at half-maximum (FWHM) of 1.3 ps. The FWHM of the pulse is estimated to be about 0.9 ps assuming a Gaussian pulse shape.
5. Polarization dependence of self-TPA in Si- and GaAs-PDs
Measuring the photocurrent generated in PDs is the easiest way to study the polarization dependence of self-TPA coefficient. Because the fraction of the incident photons that are absorbed is quite small, the generated photocurrent is directly proportional to the absorption coefficient
In the self-TPA measurement, only one optical beam is illuminated on a PD. The optical beam with a pulse width of 0.9 ps in the measurement setup described in section 3was used in the self-TPAmeasurement. The x- and y- axes are fixed in the laboratory frame. We consider the case when light that is linearly polarized alongthe
The anisotropy of self-TPA for linearly polarized light was measured for Si- and GaAs-PDs. The crystal axis  is made parallel to the x-axis. The linear polarization is rotated by a half-wave plate (i.e.,
whereand are the TPA coefficients for linearly polarized light polarized along the  and  directions, respectively.This parameter can be experimentallydetermined by the ratio of photocurrents.
Figures 3(a) and (b) respectively show the photocurrents generated in GaAs- and Si-PD sas a function of the rotation angle of the half-wave plate. For the GaAs-PD, the photocurrent varies with the polarization direction indicating that the TPA is anisotropic. The anisotropy parameter
The solid line in Fig. 3 (a) shows the value calculated using Eq. (15) and
Figure 4(a) and (b) respectively shows the dependence of the photocurrents generated in the GaAs- and Si-PDs on the rotation angle of a quarter-wave plate (
whereis the TPA coefficient for circularly polarized light. This parameter is estimated to be 0.1 and 0.39 from the measured photocurrents for linearly and circularly polarized light in the GaAs- and Si-PD, respectively.
The ratios and can be estimated from measured anisotropic and dichroism parameters. Table 1 lists the obtained ratios for the nonlinear susceptibility tensor elements for GaAs and Si.
This self-TPA coefficient is maximized when
The absorption coefficient for this elliptically polarized light is greater than.
The solid lines in Figs. 4(a) and (b) show the results calculated using Eq. (17) for GaAs and Si, respectively. The photocurrent shown in Fig. 4(a) reaches a maximum at
On the other hand, the photocurrent generated in the Si-PD is maximized when the angle is 0 and the incident light is linearly polarized, which contrasts the situation for the GaAs PD. Because the anisotropy parameter is small, Eq. (18) does not hold at any rotation angle
6. Discussion of self-TPA polarization dependence
The polarization dependence of self-TPA is strongly dependent on the crystal symmetry and the band structure. Hutchings and Wherettcalculated nonlinear susceptibility tensor elements based on kp perturbation . The ratios listed in Table 1 are consistent with their results. Murayamaand Nakayama have performed
|Anisotropy parameter ||-0.45|
|Dichroism parameter ||0.1||0.39|
It is very reasonable that GaAs and Si were observed to have quite different anisotropies because of their different crystal symmetries and band structures. As GaAs has a direct transition type band structure, an optical transition occurs at around the Γ point. The anisotropy for GaAs is due to the allowed–allowed transition [7,9] (see Fig. 5(a)), which is the two-step optical transition of. Γ15
On the other hand, Si has the indirect transition type band structure. Figure 5(b) schematically shows the band structure and the irreducible representation of this space group [11,12]. A photon energy of 0.8 eV is too small to induce a direct TPA transition without phonon absorption or emission at any point in the first Brillouin zone of Si. The final sate of the TPA transition is Δ1, which has the minimum energy of the conduction band. Many complicated transitionsequences that include optical and phonon transitions exist to reach the final point Δ1 for electron.
When both optical transitions occur at Γ point, an electron is scattered to Δ1in the conduction band. However, two step optical transitions in Si are quite different from that in GaAs. Si crystal has a point group of
When a phonon process occurs after the first optical transition, the polarization effect of the first optical transition on the intermediate state of TPA can be destroyed by the phonon process. The anisotropy is thus considered to be reduced by this process.
7. Cross-TPA in Si-APD
As shown in the previous section, TPA in Si is isotropic. Thus, TPA in Si-PD is simpler than that in GaAs-PD. In addition, a Si avalanche photodiode (APD) with the multiplication gain is commercially available. Consequently, we concentrate on cross-TPA in Si-APD.
Cross-TPA depends on the relationship between the polarization vectors of the two beams. We measure three cases: when both beams are linearly polarized, when one optical beam is linearly polarized and the other is varied between linear, elliptical, and circular polarization by a quarter-wave plate, and when one beam is circularly polarized and the other is varied between linear, elliptical, and circular polarization .
Figure 6 shows the photocurrent when both beams are linearly polarized. The horizontal axis of the figure is the rotation angle of the half- wave plate. The photocurrent was normalized using the minimum photocurrent. The photocurrent is strongly dependent on the orientation of the two linear polarization axes and has a maximum and minimum values when the polarization axes of the two optical pulses are parallel and perpendicular, respectively.
Equation (8) can be written as
The absorption coefficient hasa maximum and minimum whenand are parallel and orthogonal, respectively. The ratio of the maximum to minimum values is
Figure 7 shows the photocurrent when one beam () was linearly polarized and the polarization of the other beam () was varied using a quarter-wave plate. The horizontal axis is the rotation angle of the quarter-wave plate. The polarization of the second beam varied between linear, elliptical, and circular in this case. The solid line shows the calculated value using the parameters in Table 1. The photocurrent had maximum and minimum values when the second beam was linearly and circularly polarized, respectively. The ratios are theoretically written as
Figure 8 shows the photocurrent when one beam was circularly polarized while the polarization of the other beam was varied using a quarter-wave plate between linear, elliptical, and circular polarization. The unit vectors for circular polarization are and. An arbitrary polarization vector can be written as a linear combination of these unit vectors.
When, Eq (8) can be written as
We used the relations and.
The dependence of the absorption coefficient on the rotation angle is
The ratio of the maximum tominimum values is
It is estimated to be 1.13 using the parameters in Table 1.
The solid line in Fig. 8 is the calculated results using Eq. (24). When one optical beam is circularly polarized, cross-TPA exhibits very weak dependences on the polarization of the other optical beam because values of and are very close to each other: however, in principle, they are not exactly equal for Si.
The calculated values shown by the solid lines in Figs. 6, 7, and 8 are obtained by nonlinear susceptibility tensor elements that were deduced from the polarization dependence of self-TPA. There is relatively good agreement with the measured cross-TPA. This demonstrates that the polarization dependences of self- and cross-TPA of Si are consistent with theoretical analysis based on the nonlinear susceptibility tensor.
8. Polarization measurement by cross-TPA
The polarization dependence of the cross-TPA in Si-APD can be used to measure the polarization. In this method, a Si-APD is irradiated by the arbitrarily polarized light to be measured (signal light) and a linearly polarized referencebeam. The photocurrents generated by cross-TPA between the signal light and the linearly polarized reference light are measured. Polarization direction of the reference beam was varied in four ways. Polarization of the arbitrarily polarized light can be determined from the four photocurrents of the APD .
Several applications require the ability to detect rapid variations in the polarization of an optical signal. In all conventional polarization measurement methods, the temporal resolution is limited by the response speed of the PD and/or electrical devices employed. Measurements based on TPA can be employed to measure rapidly varying polarization without the need to use high-speed electronics. Since the reference beam can be short pulses, the temporal polarization of a short-time period can be measured using this method. The temporal resolution is limited by only the pulse width of the reference light.
8.1. Principle of polarization measurement
The polarization of thelight to be measured can be generally described by the Jones vector
Let us consider four different polarization orientations of the linearly polarized reference light beam, namely,
Thus, the parameters of the measured light are given by
The polarization can be determined from the ratios of the photocurrent
Let us consider the case when the pulse width of the reference light is much shorter than that of the light to be measured. The measured photocurrent produced by APD due to cross-TPA samples the polarization of the light being measured during the reference light pulse. It is thus possible to measure polarization as a function of time by varying the timing of the short reference light pulse.
One problem with this measurement method is that the sign of cannot be determined as long as the reference light is linearly polarized. When it is important to determine the sign of, it is necessary to compare the two photocurrents generated by cross-TPA for right and left circularly polarized localized lights; let us define these absorption coefficients as
respectively. The sign of is positive when
8.2. Measurement of stationary polarization
Polarization measurements were performed using the same setup as that shown in Fig. 1. The reference light is linearly polarized and its polarization direction
Figure 9 shows the measured elements of the Jones vector of the light being measured. The circles and triangles represent the measured points, while the soid lines represent the theoretical curves given by Eq. (13). Figure 9(a) shows the amplitudes of
The light to be measured is circularly polarized () at
8.3. Measurement of time-dependent polarization
The instantaneous polarization when the two light pulses overlap was measured for the cross-TPA. It is thus possible to measure the time-dependent polarization without using high-speed electronics using this method. An optical pulse compressed to 0.9 ps was used for the local oscillation in this measurement. The time resolution is equal to the width of this pulse. The timing of the short reference pulse was scanned over the signal light pulse to trace the variation of the polarization of the signal light pulse.
The polarization of the light being measured was varied with time using a polarization-maintaining fiber. The output of the gain-switched LD was made linearly polarized and its polarization direction was tilted at an angle of 45° relative to the fast and slow axes of the fiber. The propagating optical pulse was separated by the birefringence of the polarization-maintaining fiber since components polarized along the two axes have different the propagation velocities. Consequently, the polarization of the output optical pulse was made time-dependent. A 20-m-long polarization-maintaining fiber imparted a propagation time difference of about 30 ps between the two components.
Figure 10 shows the Jones vector of the output pulse of a polarization maintaining fiber. The x- and y-axes are parallel to the fastand slow axes, respectively. Figure 10(a) shows the measured amplitudes
8.4. Measurement of wavelength chirping
The measured phase difference
As the optical frequency is shifted by the wavelength chirping during the time period of
whereis the wavelength chirping gradient. The output pulse propagates in free space for a length of
The light is, therefore, converted into elliptically polarized light.
Figure 11 shows the measured wavelength chirping of an optical pulse from a gain-switched LD. The linearly polarization is tilted at 45° relative to the principal axis of the 7λ/4 wave plate. The optical pulse passes through the wave plate and propagates in 40-cm of free space.
The chirping gradient is shown by the left vertical axis in Fig. 11. The measured value is consistent with the wavelength broadening observed by an optical spectrum analyzer. The chirping gradient is large at the head of pulse due to the asymmetry pulse shape.
8.5. Measurement of dynamic birefringence of a semiconductor optical amplifier
Semiconductor optical amplifiers (SOAs) generally exhibit birefringence due to the real and/or imaginary parts of the optical gain having different values for transverse electric (TE) and the transverse magnetic (TM) polarizations. The real and imaginary parts of the SOA gain are nonlinear for intense propagating light and induce dynamic birefringence [15,16]. Intense optical pulse affects the polarization of the pulse itself. Consequently, polarization of the output pulse from a SOA varies with time.
A linearly polarized signal light was injected into a SOA witha polarization direction tilted at 45 ° against TE and TM modes. Time dependent Jones vector components of the output pulse from the SOA are measured by the cross-TPA with a reference light pulse with a pulse width of 0.9 ps. The results are shown in Figs. 12(a) and (b). The closed circles and triangles in Fig. 12(a) show the measured amplitudes
Photocurrents generated by TPA in PDs were studied. The ratios of nonlinear susceptibility tensor elements were deduced from the polarization dependence of self-TPA for Si- and GaAs-PDs. The photocurrent was isotropic for linear polarization in the Si-PD. On the other hand, TPA is anisotropic and the photocurrent depends on the linear polarization direction in GaAs-PD. The photocurrents for elliptically and circularly polarized light can also be analyzed by the imaginary parts of the nonlinear susceptibility.
The polarization dependence of cross-TPA was measured for a Si-APD. Three types of cross-TPA that are linear-linear, linear-elliptic, and circular-elliptic polarizations were studied. The measured results agree with theoretical values calculated by using parameters obtained from the polarization dependence of self-TPA. These results demonstrate that both self- and cross-TPA can be well described by analysis based on the nonlinear susceptibility tensor.
Cross-TPA was applied to polarization measurements. The Jones vector elements of anarbitrarily polarized signal light can be determined from the four photocurrents generated by cross-TPA between the signal light and the linearly polarized reference light. The time resolution is limited only by the pulse width of the reference light pulse. This measurement method can thus be used to detect rapid polarization variation. It was demonstrated that the polarization of a light pulse from a polarization-maintaining optical fiber and a SOA can be measured by this method.