Generating Clean 10-Microsecond Pulses for VCSELs in 3D Sensing

For precision-based 3D sensing systems, generating short, stable pulses is essential. In applications like Vertical Cavity Surface Emitting Lasers (VCSELs), used in depth-sensing technologies, generating clean 10-microsecond pulses with minimal distortion is critical for accuracy. Keithley Instruments offers the 2601B-PULSE SMU, leveraging PulseMeter™ technology that delivers consistent 10 µs pulses at high currents (up to 10 A) and voltages (up to 10 V). This advanced SMU technology addresses common challenges like cable inductance and oscillation, supporting accurate and efficient VCSEL pulse generation.
 

When transmitting fast signals to a Device Under Test (DUT), coaxial cables are typically used due to their shielding and performance capabilities. However, inductance within these cables can critically affect pulse generation, leading to oscillations or distorted pulses that make 3D sensing measurements unreliable.

 

Key factors affecting cable inductance

• Center Conductor Diameter: Thicker conductors generally reduce inductance.
• Distance to the Outer Shield: Closer proximity between the center conductor and shield can decrease inductance.
• Cable Length: Cable inductance is directly proportional to length, often reaching up to 2 µH per meter in typical setups.

Addressing Cable Inductance

The Keithley 2601B-PULSE SMU effectively minimizes these inductance issues through both internal and external configurations:

• Parallel Coaxial Cables:
  By tying the shields of two coaxial cables together at both ends, total inductance can be reduced to around 500 nH—a significant improvement for clean pulse delivery.
• SC-182 Low Inductance Cables:
  Keithley’s SC-182 cables are optimized for high-speed pulse applications, offering less than 150 nH per meter of inductance. In setups using two of these 1-meter cables with the shields shorted, inductance remains under 300 nH, reducing potential pulse distortion.
• Standard Coaxial Cables:
  For standard 50 Ω coaxial cables with about 350 nH per meter inductance, shorting the shields at both ends similarly helps in lowering inductance, achieving cleaner pulses.

Oscillations within a pulse make it difficult to measure accurately, as the signal takes longer to settle, impacting precision in 3D sensing. To manage oscillations, some pulse-generating instruments compromise with slower rise times to stabilize the pulse. However, this leads to longer settling times, delaying results.

The Keithley 2601B-PULSE SMU solves these issues by supporting pulse edges with peak voltages needed to energize and de-energize the inductance quickly, preventing slow rise times and maintaining stability within microseconds. With this approach, the 2601B ensures that pulses stabilize rapidly, providing high accuracy for fast 3D measurements.

Key benefits include:

• Load Tolerance: The 2601B supports load-cable resistances up to 20 Ω and inductances up to 3 µH. This ensures pulse edges remain virtually unchanged, regardless of load-cable resistance variations within this range.
• Stable Pulse Shape: Even if the load resistance changes, the pulse shape stays fast and clean, avoiding the need for constant recalibration.
• Pulse Width, Current, and Voltage: The 2601B can generate pulses at widths as short as 10 µs, with up to 10 A current and 10 V voltage, achieving clean pulses as long as total inductance (including DUT and cables) remains within 3 µH.

Figure 4 shows the type of current amplitude sweep typically used for LIV measurements, offering a consistent rise time and pulse width for each current level. When compared with Figure 3, Figure 4 shows a flat, consistent pulse width for each level and a consistent rise time, which allows for a stable, more consistent optical measurement because current change is directly related to changes in optical power and light intensity.

Figure 6 shows the results of another experiment made with various load resistances from 1 Ω to 10 Ω with a 1 A, 10 µs pulse. Given that a resistance of 10 Ω is quite a high impedance, it would normally slow the rise time, much as is shown in Figure 5; here, however, all the waveforms are most likely identical, like a single waveform.

The Keithley 2601B-PULSE SMU integrates a traditional source measure unit's functionality with advanced pulse generation capabilities. For engineers working with VCSELs in 3D sensing, it offers the following advantages:

• Fast, Clean 10 µs Pulse Generation: Achieves precise, undistorted pulses with minimal rise times, even at high currents and voltages.
• Reduced Inductance Impact: Through SC-182 low-inductance cables and optimized configurations, inductance is effectively managed, improving signal clarity.
• No-Tuning Precision: Operates seamlessly across various resistance and inductance levels, ensuring stability without complex tuning.

With the Keithley 2601B-PULSE, engineers can achieve reliable, high-performance pulse generation crucial for advanced 3D sensing applications. Its design not only simplifies setup but also optimizes pulse quality, enhancing depth accuracy for VCSEL-driven systems.

Interested in Keithley 2601B-PULSE SMU? Visit our Keithey 2601B webpage for more product details:
Contact us for more information like pricing, demonstrating session or even schedule an online meet up! 

Also Find out more: 

 More Keithley Products

  More SMU Products.