Optical and electrical characteristics and testing of 40Gb/s WD-PIN-PD/TIA components

1. Introduction

For a single-channel 40Gb/s optical transmission system, its optical detector generally cannot adopt a PIN-PD structure with front light input. This is because the PN junction capacitance and stray capacitance of such a detector with front light input are large, and the carrier transit time is long, which limits its optical response rate (or transmission bandwidth). In order to improve the optical response rate (bandwidth), the detector adopts a narrow strip waveguide PIN structure with side light input (WD-PIN-PD). The PN junction capacitance of this structure can be less than 80ff (1ff = 10-15f), and the stray capacitance is reduced by using a semi-insulating substrate; the carrier transit time is reduced by reducing the thickness of the light absorption region. These structural changes make the optical response bandwidth of the waveguide PIN optical detector greater than 30GHz, and some even reach more than 50GHz.

In 2003, we began to design and trial-produce 40Gb/s WD-PIN-PD. After nearly two years of practice, we have successfully produced 40Gb/s WD-PIN-PD. The measurement results show that the dark current of the detector is less than 15nA, the light responsivity can be greater than 0.46A/W, and the -3dB analog bandwidth reaches 32GHz. When it is assembled with a 40b/s TIA, the light receiving sensitivity can reach -7dBm.

2.40Gb/s WD-PIN-PD structure

40Gb/s WD-PIN-PD has several different structures. We designed a quasi-coplanar waveguide (CPW)-side-light waveguide detector integrated structure, the schematic diagram of which is shown in Figure 1.

Figure 1 Schematic diagram of 40Gb/s WD-PIN-PD structure with CPW

Figure 1 shows the integrated structure of CPW output and WD-PIN-PD. The core of this structure is 40Gb/s WD-PIN-PD. It consists of Fe-doped semi-insulating InP substrate, n+-InP transition layer, n-InP optical matching layer, n-In0.52Al0.48As optical waveguide layer, i-In0.53Ga0.47As optical absorption layer, P-In0.52Al0.48As optical waveguide layer, P+-InP optical matching layer and P+-In0.53Ga0.47As contact layer, where the thickness of i-In0.53Ga0.47As optical absorption layer is 0.52-0.58μm.

In order to form a mirror-like light-incoming surface, it is first made into a double-table twin-core structure, as shown in Figure 2, and finally a single tube core is formed through cleavage technology. This twin-core structure has been applied for a national patent. The double-table twin-core structure is shown in Figure 2.

Figure 2 Schematic diagram of the double-table twin core structure

3. Photoelectric characteristics of 40Gb/s WD-PIN-PD

The photoelectric characteristics of 40Gb/s WD-PIN-PD include IV characteristics, wavelength response characteristics, photoelectric conversion characteristics, switching characteristics, etc. It can be measured by technical indicators such as photocurrent, dark current, breakdown voltage, PN junction capacitance, wavelength response range, photoresponsivity, -3dB bandwidth, relative intensity noise, etc. The key technical indicators for 40Gb/s WD-PIN-PD are dark current, -3dB bandwidth and photoresponsivity. [page]

3.1 Photocurrent and dark current

Photocurrent is the current that flows in the external circuit when the active region (also called active region) of the detector absorbs light to generate carriers, drifts or diffuses under high field, and then generates additional potential in the PIN depletion region. This current is closely related to factors such as incident light power, optical coupling efficiency, incident surface reflection coefficient, detector photosensitive surface area, light absorption coefficient of the absorption layer material in the detector, and internal quantum efficiency. According to the continuity equation and boundary conditions, the photocurrent Iopt can be expressed as:

Iopt = (1- R) Iopt (x) dx = (1-R) ​​Iopt (0) e(-α(λ)x) dx (1)

Here, R is the reflection coefficient; α(λ) is the light absorption coefficient. For InGaAs, α(λ)≈ (2-4) ×103 cm-1;

e-α(λ) reflects the utilization rate of the incident light. When the length of the active region L> 1/α(λ), the utilization rate of light can reach more than 95%; Iopt (0) is the initial photocurrent. It is closely related to the internal quantum efficiency in the active region of the detector and is a sensitive function of the material structure parameters.

Dark current refers to the current of the detector when there is no light at a specified reverse voltage. It is a noise limit on the light receiving sensitivity of the detector. It is also the main indicator for measuring the quality of the photodetector manufacturing technology. This current is mainly composed of the generation current in the PIN junction depletion region, the diffusion current in the PIN junction adjacent region, the tunnel current in the I region, and the surface leakage current. For mesa-type devices, the protection of the exposed PIN mesa is important. If the exposed PIN junction around the mesa is not well protected, the surface leakage current will be large, and even become the main component of the dark current. Generally, the dark current of a planar PIN-PD with a photosensitive surface diameter of 45μm is generally less than 1nA, while the dark current of a mesa-type PIN-PD with the same area is generally greater than 10nA. The dark current of the 40Gb/s WD-PIN-PD we made is in the range of 0.2nA to 20nA at -3.3V.

3.2 Photoresponse rate and -3dB bandwidth

The signal transmitted in the digital communication optical fiber is a digital optical signal. For the reception of 40Gb/s optical signals, the optical detector must have the ability to track the signal at high speed, and its tracking ability is the photoresponse rate. According to the Fourier time-frequency domain transform, the photoresponse rate can be characterized by the -3dB bandwidth.

The photoresponse rate is limited by factors such as carrier transit time and RC time.
The carrier transit time is related to the reverse bias voltage, carrier saturation drift velocity, and transit region thickness. There are two types of photogenerated carriers: electrons and holes, and their saturation drift velocities are different. The relationship between the steady-state electron and hole saturation drift velocity and the electric field in In0.53Ga0.47As is shown in Figure 3.

Figure 3 Relationship between steady-state electron and hole drift velocity and electric field

Figure 3 shows that when the electric field strength reaches 5× 104 V/cm or more, the electrons and holes reach a steady-state saturation drift velocity of 2×106 Cm/s respectively. If the thickness of the transition zone is 0.58μm and the electric field strength is E = 5× 104 V/cm, the required external reverse bias voltage is 2.9 v. At this time, the hole transition time τP is less than 29ps and the electron transition time τn is less than 10 ps.

The RC time of the 40Gb/s WD-PIN-PD can be represented by the parameter value RC of its small signal equivalent circuit. The small signal equivalent circuit of the 40Gb/s WD-PIN-PD is shown in Figure 4:

Figure 4 40Gb/s WD-PIN-PD small signal equivalent circuit

In Figure 4, Iph is the equivalent current source of the 40Gb/s WD-PIN-PD, RJ is the equivalent internal resistance of the PD in reverse position, usually above 50 MΩ; Cj is the PIN junction capacitance, generally less than 80f F; CP is the stray capacitance, which is usually omitted; Rs is the PD equivalent series contact resistance, generally less than 15Ω; RL is the load resistor; Ls is the equivalent inductance of the interconnection line. For 40Gb/s WD-PIN-PD, RC can be expressed as:

RC ≈ (Rs + RL + jωLs) Cj = ε0εr (Rs + RL + jωLs) A / w (2)

Here, A is the PN junction area of ​​the PD, and w is the thickness of the light-active region. (2) shows that to reduce the RC time, Rs, RL and A should be minimized as much as possible; the purpose of using a PIN-PD with side light entry is to reduce A and w.

It should be noted that for 40Gb/s WD-PIN-PD, Ls has a great influence on the transmission characteristics and bandwidth. For a 100μm long line, the impedance will reach 20-30 (it is related to the thickness and shape of the line), and the inductance will reach 30-40nH. Therefore, it is very important to minimize the length of the interconnection line for both bandwidth and transmission loss.

The bandwidth Δf-3dB of 40Gb/s WD-PIN-PD can be estimated by measuring its pulse front rise time tr.

Δf-3dB and tr have the following approximate relationship:

Δf-3dB (GHz) ≈ 0.35 / tr (ns) (3)

If tr < 11 ps, then Δf-3dB > ≈31 GHz.
Here, Δf-3dB can be expressed as:

Δf-3dB ≈ 1/[2π( RC)2 + τn2 ]]-2 (4)

Here τn is the electron transit time. It is worth noting that both RC and τn are related to the thickness w of the carrier transit region. The larger W is, the smaller the PIN junction capacitance is, while the carrier transit time is longer. This is contradictory. Therefore, the selection of W should be considered in a compromise. [page]
3.3 Photoresponsivity

Another important indicator for measuring 40Gb/s WD-PIN-PD is photoresponsivity (Re), which is the ratio of the generated photocurrent to the incident light power. Photoresponsivity is not only related to the absorptivity, absorption length, internal quantum efficiency, incident surface reflectivity of the absorption layer material, but also closely related to the distance of the PN junction from the surface and the fiber coupling efficiency. For the 40Gb/s WD-PIN-PD with only a 0.5×6μm2 light-incoming surface, the fiber coupling efficiency and fiber positioning are the most critical issues.

We designed and fabricated a wedge-shaped optical fiber, using a specific gasket and a specific curing glue, so that the photocurrent of the 40Gb/s WD-PIN-PD reaches 95-120μA at 200μW optical power.

3.4 40Gb/s PIN-PD-TIA component optical receiving sensitivity

The 40Gb/s WD-PIN-PD and 40Gb/s TIA are finely assembled through coplanar waveguide, and the 40Gb/s PIN-PD-TIA component is completed. After preliminary testing, the optical receiving sensitivity of the component can reach -7dBm.

4. Photoelectric characteristic test

We measured the PIN-TIA component for 40Gb/s SDH optical fiber communication equipment through the Hubei Provincial Electronic Product Quality Inspection and Supervision Institute and the Institute of Microelectronics of the Chinese Academy of Sciences (test S21). The test results are shown in Table 1.

Table 1 PIN-TIA component test results for 40Gb/s SDH optical fiber communication equipment

The dark current and responsivity measurement device of 40Gb/s WG-PIN-PD is shown in FIG5 .

Figure 5: Dark current and responsivity measurement device of 40Gb/s WG-PIN-PD

Conclusion

Through nearly two years of unremitting efforts, we have designed and produced a 40Gb/s CE side-input waveguide photodetector with our own intellectual property rights. Preliminary tests show that the dark current of the photodetector is generally less than 15nA at -3.3V, the photoresponsivity is greater than 0.45A/W, and the analog electrical bandwidth at -3dB can reach 32GHz.

Author unit: Wuhan Telecom Devices Co., Ltd.
Acknowledgements In the development of 40Gb/s WD-PIN-PD, Luo Biao, Zhou Peng, Wang Jin, and Wang Fei participated in the process production, and Zhang Xuejun and Lei Cheng carried out the die installation, optical coupling and testing. We would like to express our sincere gratitude.

References
John E. Bowers and Charles A .Burrus, Ultrawide-Band Long-Wavelength PIN Photodetectors, J. of Lightwave Technology, vol. LT-5,No. 10, pp.1339-1350, October 1987.