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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 16 — Aug. 3, 2009
  • pp: 14389–14394
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Behaviors of the third order intercept point for p-i-n waveguide photodiodes

Meredith N. Draa, Jeffrey Bloch, David C. Scott, Nong Chen, Steven Bo Chen, William S. C. Chang, and Paul K. L. Yu  »View Author Affiliations


Optics Express, Vol. 17, Issue 16, pp. 14389-14394 (2009)
http://dx.doi.org/10.1364/OE.17.014389


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Abstract

Waveguide PIN photodiodes with different absorber thicknesses and lengths were fabricated and characterized for linearity. Device A has a thicker absorber and shorter length, resulting in a bandwidth of 20GHz while device B reduces the absorber by half while maintaining the intrinsic layer thickness and almost doubles the length, resulting in a smaller optical overlap factor and a bandwidth of 10GHz. Device B shows a significant enhancement in OIP3 with a record high maximum value for a PIN waveguide photodiode of 42.4dBm at 28mA and -4V bias compared to device A which has a maximum OIP3 of 32.7dBm at 10mA and -4V bias. The increased linearity in device B is attributed to the reduction in optical overlap factor and increase in device length resulting in an easing of the front facet photocurrent density and overall device heating. The DC saturation points are about 75mA and >160mA for device A and B at -2V bias.

© 2009 Optical Society of America

1. Introduction

Additionally, WGPDs have been analyzed over a range of frequencies for OIP3 [1

1. H. Jiang, D. S. Shin, G. L. Li, T. A. Vang, D. C. Scott, and P. K. L. Yu, “The frequency behavior of the third-order intercept point in a waveguide photodetector,” IEEE Photon. Technol. Lett. 12, 540–542 (2000). [CrossRef]

]. Previously reported is the flat response of OIP3 for a 20GHz PIN WGPD that will be revisited here and compared to a new design [7

07. M. Draa, J. Ren, D. C. Scott, W. S. C. Chang, and P. K. L. Yu, “Frequency behaviors of the third order intercept point for a waveguide photodiode using a three laser two-tone setup,” [Conference Paper] LEOS 2008, 284–285 (2008).

].

2. Device fabrication and design

Fig. 1. Device design parameters
Fig 2. Material structure of the layers with bandgap wavelength in parenthesis.

The effects on OIP3 are studied using two devices with similar layer structures designed for 20GHz (device A) and 10GHz (device B) bandwidth. The factor of importance is the reduction in the optical overlap factor for the 10GHz device. For Device B, the absorber is made thinner, but the intrinsic layer thickness is maintained the same to examine the effects of reducing the heating at the front of the device and absorption along the length of the device. Additionally, the longer device is necessary to maintain similar responsivity for each device since the overlap factor (Γ) is significantly reduced in device B. Figure 1 shows the design parameters for each device. The layer structures are shown in Fig. 2 where X corresponds to the absorber layer thickness detailed in Fig. 1. The length of device B is almost twice as long as device A because of the capacitance requirements for the bandwidth design and to maintain responsivity.

3. Simulation

Both devices were modeled using Silvaco International’s numerical device simulator ATLAS to solve for the DC responsivity curve. The result can be seen in Fig. 3. The simulator is 2D with top down illumination. Since both devices are waveguide style, the output is scaled to 3D using the device geometry and the optical overlap factor. The simulation does not take into account the thermal heating that occurs in the device and has not been calibrated with measured data. The results indicate a higher expected 1dB DC saturation for device B. Later results will demonstrate the beneficial behavior of the device in terms of linearity.

P(z)=PinW·DqhvVbΓαηexp(Γηz)(W/m3)
(1)

where Pin is the input optical power, W is the width of the mesa, D is the height of the intrinsic region, q is the electronic charge, h is Planck’s constant, v is the frequency, Vb is the bias voltage, Γ is the optical overlap factor, α is the absorption coefficient and η is the input optical coupling efficiency. The device is simulated over a range of input power and the maximum temperature in the device is recorded. The output photocurrent can be found by integrating Eq. (1) over the intrinsic region and dividing by the bias voltage:

Ipd=Pinqhvη(1exp(ΓαL))(A)
(2)

where L is the length of the device. Using data from device A the output photocurrent is scaled to approximate that the device fails at 600K. From this, device B can be compared in measurement to assess whether the model predicts the failure point. The results of the simulation can be seen in Fig. 4. The simulation predicts that device B will fail at slightly twice the amount of photocurrent of device A.

Fig. 3. Simulation of DC responsivity for device A and B.
Fig. 4. Simulation of heating in device A and B at -4V bias.

4. Results and discussion

The devices were measured for bandwidth using an Agilent 86030A Lightwave Analyzer. The bandwidth measurements were made up to 50GHz at -4V bias and 1mW input optical power and can be seen in Fig. 5 and Fig. 6 for device A and B respectively. The graphs show measurements for many of the same devices to demonstrate the consistency of the bandwidth. The bandwidths are 20GHz and 10GHz for device A and B. Additionally, the device photocurrent failure point was measured at -4V bias with a result of 32mA and 49.3mA for device A and B. Device B exhibits almost two times the current capability of device A at -4V bias due to the reduction in optical overlap and the increased length of the device.

The devices were measured for DC saturation at -2V bias with a responsivity of .5A/W and can be seen in Fig. 7 along with the simulated DC saturation curves performed in Silvaco. The maximum recorded responsivity of each device is .75A/W and .74A/W for device A and B respectively. The saturation point was determined by recording output DC photocurrent versus input DC optical power and finding a linear line of best fit at low input power to determine the approximate photocurrent where DC saturation begins. Device A begins to saturate at about 75mA while device B did not saturate, but exhibited thermal runaway and failed at 160mA. The simulation was scaled according to device geometry parameters as detailed above and then calibrated to the results. The simulation does not predict saturation as in the 4V bias case for device B seen in Fig. 3. In Silvaco, the electric field is observed to collapse which should induce saturation. Instead a large increase in electron current density occurs in the p-region overtaking the hole current density, which leads to recombination near the intrinsic region to p-region interface. The recombination reduces the carrier densities and an induced electric field is observed in the intrinsic region. The field across the intrinsic region increases the carrier transport to their saturation velocities causing the runaway current observed in the measurement.

The device OIP3 was measured using the three laser two-tone setup in [8

8. M. Draa, J. Ren, D. C. Scott, W. S. C. Chang, and P. K. L. Yu “Three laser two-tone measurement of photodiode intercept points,” Opt. Express 16, 12108–12113 (2008). [CrossRef] [PubMed]

]. Two distributed-feedback lasers were externally modulated and amplified with an erbium doped fiber amplifier (EDFA). The EDFA is held at constant power and the output at each is attenuated using a variable optical attenuator (VOA) and then combined with a 50/50 coupler. A third un-modulated distributed feedback laser is amplified with an EDFA and also controlled by a VOA. The two branches are combined with a wavelength division multiplexer (WDM) and input into the device. The measurements were made as discussed in [8

8. M. Draa, J. Ren, D. C. Scott, W. S. C. Chang, and P. K. L. Yu “Three laser two-tone measurement of photodiode intercept points,” Opt. Express 16, 12108–12113 (2008). [CrossRef] [PubMed]

] where the slope is three to ensure third order distortion at the given frequency is due to the detector alone.

Fig. 5. Normalized frequency response of device A up to 50GHz.
Fig. 6. Normalized frequency response of device B up to 50GHz.

IMD3 for each device was measured with frequency tones of 1GHz and 1.1GHz. The results can be seen in Fig. 8 with the fundamental and IMD3 plotted for both devices. The devices are biased at -4V and have a DC photocurrent of 10mA and 28mA for device A and B respectively. The OIP3 in Fig. 8 is 30.5dBm and 42.4dBm for device A and B. In the graph both IMD3 sets of data exhibit a slope of 2.97 based on the trend line which is within measurement error. The devices were measured for OIP3 versus a number of different variables for comparison.

Fig. 7. DC saturation measurement and simulation (black) for device A (blue) and B (red) at -2V bias
Fig. 8. IMD3 for device A and B with -4V bias

In Fig. 9 the OIP3 of the devices are measured over a range of photocurrents with data for device A in blue and device B in red at input frequencies of 1GHz and 1.1GHz and bias voltage of -4V. Device A shows an increase in OIP3 up to 10mA and then is relatively flat until its failure point at -4V bias which is usually slightly more than 30mA. The coupling for device B was optimized for the best nonlinearity possible for the particular device. Two sets of data shown for device B are for two different devices to demonstrate the high reliance of OIP3 on fiber coupling. The circle data shows a significant increase in OIP3 over photocurrent and overall is between 5 to 10dBm higher than device A with a peak occurring at 28mA. The triangle data shows the same increase in OIP3 as photocurrent increases but a flatter response from 20mA to 30mA. During measurements a specific fiber position could be tweaked to see approximately a 10dBm reduction in IMD3 with little effect on the fundamental thereby significantly increasing OIP3. In device A, the fiber positioning was not observed to have a significant effect on OIP3 while maintaining a consistent responsivity. Because of the difference in device structures, there may be a difference in the excitation and propagation of the optical modes which would lead to different thermal distributions and possibly generated distortions that could make OIP3 of device B more sensitive to the fiber position.

Fig. 9. OIP3 for device A and B with -4V bias at 1GHz.
Fig. 10. OIP3 for device A and B at 15mA and 20mA at 1GHz.

Both devices were measured over a range of bias voltages which can be seen in Fig. 10. The purpose of this measurement was to determine the optimal bias point of the device when considering linearity and thermal heating tradeoffs. At a higher bias the device may have better linearity, but will dissipate more power. For device A, the measurements were taken at 15mA photocurrent from -2V to -6V. The device failed at -6V and at about 90mW of power. For device B, the measurements were taken at 20mA from -2V to -9.5V, where the device failed at about 190mW of power, which is almost double that of device A. In Fig. 10, the OIP3 shows an increase from -2V to -4V and then a leveling off for both devices, with device B about 8dBm higher for OIP3. Device B however shows an increase in OIP3 from -7V to -9.5V with a peak of 40.5dBm for OIP3 at -9.5V. Both devices were designed to operate at -4V, in order to maximize linearity by maintaining a particular electric field across the device that is below the breakdown voltage but greater than the point at which carrier velocities are no longer constant. From the results is the desired bias operation is confirmed with the leveling off of OIP3 at voltages higher than -4V.

Fig. 11. OIP3 for device A at 10mA and B at 25mA and -4V bias

5. Conclusion

Two WGPD devices were fabricated and characterized demonstrating the effects of reducing the optical overlap factor and increasing thermal capacity through lengthening the device, with the tradeoff of bandwidth. The devices were characterized first through two simulation programs for output DC saturation and thermally for device maximum photocurrent. The 1dB DC saturation points were measured as 80mA and >150mA for device A and B. Device B showed a high dependence of OIP3 on fiber positioning due to the decreased absorber thickness. A significant enhancement of OIP3 and power capability is observed for device B with a record high maximum OIP3 of 42.4dBm at 28mA and 1GHz frequency for a PIN waveguide photodiode.

Acknowledgements

The authors would like to acknowledge Prof. Sadik Esener for the loan of RF spectrum analyzer. This work was supported by DARPA STTR program and DARPA/SPAWAR program N66001-03-8938 TDL46, both under Dr. Ron Esman.

References and links

1.

H. Jiang, D. S. Shin, G. L. Li, T. A. Vang, D. C. Scott, and P. K. L. Yu, “The frequency behavior of the third-order intercept point in a waveguide photodetector,” IEEE Photon. Technol. Lett. 12, 540–542 (2000). [CrossRef]

2.

D. C. Scott, T. A. Vang, J. Elliott, D. Forbes, J. Lacey, K. Everett, F. Alvarez, R. Johnson, A. Krispin, J. Brock, L. Lembo, H. Jiang, D. S. Shin, J. T. Zhu, and P. K. L. Yu, “Measurement of IP3 in p-i-n photodetectors and proposed performance requirements for RF fiber-optic links,” IEEE Photon. Technol. Lett. 12, 422–424 (2000). [CrossRef]

3.

H. Jiang and P. K. L. Yu, “Waveguide integrated photodiode for analog fiber-optic links,” IEEE Trans. Microwave Theory Tech. 48, 2604–2610 (2000). [CrossRef]

4.

A. Beling, H. Pan, H. Chen, and J. C. Campbell, “Linearity of modified uni-traveling carrier photodiodes,” J. Lightw. Technol. 26, 2373–2378 (2008). [CrossRef]

5.

J. Klamkin, A. Ramaswamy, N. Nunoya, L. A. Johansson, J. E. Bowers, S. P. DenBaars, and L. A. Coldren, “Uni-traveling-carrier waveguide photodiodes with >40dBm OIP3 for up to 80mA of photocurrent,” IEEE J. Quantum Technol. 44, 354–359 (2008). [CrossRef]

6.

S. Jasmin, N. Vodjdani, J. Renaud, and A. Enard, “Diluted- and distributed-absorption microwave waveguide photodiodes for high efficiency and high power,” IEEE Trans. Microwave Theory Tech. 45, 1337–1341 (1997). [CrossRef]

07.

M. Draa, J. Ren, D. C. Scott, W. S. C. Chang, and P. K. L. Yu, “Frequency behaviors of the third order intercept point for a waveguide photodiode using a three laser two-tone setup,” [Conference Paper] LEOS 2008, 284–285 (2008).

8.

M. Draa, J. Ren, D. C. Scott, W. S. C. Chang, and P. K. L. Yu “Three laser two-tone measurement of photodiode intercept points,” Opt. Express 16, 12108–12113 (2008). [CrossRef] [PubMed]

OCIS Codes
(040.5160) Detectors : Photodetectors
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Detectors

History
Original Manuscript: June 8, 2009
Revised Manuscript: July 9, 2009
Manuscript Accepted: July 10, 2009
Published: July 31, 2009

Citation
Meredith N. Draa, Jeffrey Bloch, David C. Scott, Nong Chen, Steven B. Chen, William S. Chang, and Paul K. Yu, "Behaviors of the third order intercept point for p-i-n waveguide photodiodes," Opt. Express 17, 14389-14394 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-16-14389


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References

  1. H. Jiang, D. S. Shin, G. L. Li, T. A. Vang, D. C. Scott, and P. K. L. Yu, "The frequency behavior of the third-order intercept point in a waveguide photodetector," IEEE Photon. Technol. Lett. 12, 540-542 (2000). [CrossRef]
  2. D. C. Scott, T. A. Vang, J. Elliott, D. Forbes, J. Lacey, K. Everett, F. Alvarez, R. Johnson, A. Krispin, J. Brock, L. Lembo, H. Jiang, D. S. Shin, J. T. Zhu and P. K. L. Yu, "Measurement of IP3 in p-i-n photodetectors and proposed performance requirements for RF fiber-optic links," IEEE Photon. Technol. Lett. 12, 422-424 (2000). [CrossRef]
  3. H. Jiang and P. K. L. Yu, "Waveguide integrated photodiode for analog fiber-optic links," IEEE Trans. Microwave Theory Tech. 48,2604-2610 (2000). [CrossRef]
  4. A. Beling, H. Pan, H. Chen and J. C. Campbell, "Linearity of modified uni-traveling carrier photodiodes," J. Lightw. Technol. 26, 2373-2378 (2008). [CrossRef]
  5. Q1. J. Klamkin, A. Ramaswamy, N. Nunoya, L. A. Johansson, J. E. Bowers, S. P. DenBaars and L. A. Coldren, "Uni-traveling-carrier waveguide photodiodes with >40dBm OIP3 for up to 80mA of photocurrent," IEEE J. Quantum Technol. 44, 354-359 (2008). [CrossRef]
  6. S. Jasmin, N. Vodjdani, J. Renaud, and A. Enard, "Diluted- and distributed-absorption microwave waveguide photodiodes for high efficiency and high power," IEEE Trans. Microwave Theory Tech. 45, 1337-1341 (1997). [CrossRef]
  7. Q2. M. Draa, J. Ren, D. C. Scott, W. S. C. Chang and P. K. L. Yu, "Frequency behaviors of the third order intercept point for a waveguide photodiode using a three laser two-tone setup," [Conference Paper]LEOS 2008, 284-285 (2008).
  8. M. Draa, J. Ren, D. C. Scott, W. S. C. Chang, and P. K. L. Yu "Three laser two-tone measurement of photodiode intercept points," Opt. Express 16, 12108-12113 (2008). [CrossRef] [PubMed]

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