Light and Technology: What is the difference between gray light and colored light?

 We always hear about white light modules, gray light modules and colored light modules. What exactly are they and what are the differences? Let’s talk about it today. Before talking about gray light and colored light, let’s first look at what white light is.

White light is defined as: mixed light of different colors, including all colors (wavelengths) in the spectrum, such as ultraviolet light, visible light, infrared light, etc., so it has the highest brightness and is called “white light”. Gray light and colored light are also discussed in this range, but the range is in the near-infrared region. So what are the differences between gray light and colored light?

As we all know, SDH technology was the earliest used in optical transmission systems. In the optical path of SDH, one of our optical fibers only transmits one signal and one wavelength, and only one car runs in a large lane. The wavelength of this signal does not have strict requirements on the center wavelength. It only depends on whether the wavelength fluctuates within the low-loss window. The fluctuation range of the wavelength is generally around ±40nm. For example, as long as it is within the 850nm window, 1310nm window, or 1490nm or 1550nm window. That’s it. This kind of light is called gray light.

It can be said that, similar to white light, gray light also has an indefinite wavelength within a certain range, but it is not as wide as white light. Therefore, gray light is sometimes called white light, or black and white light.

Gray light is mainly defined by ITU-T G.957, ITU-T G.691, ITU-T G.959.1 and IEEE 802.3.

  • G.957 defines rate STM-1, STM-4, STM-16 and other gray optical interfaces;
  • G.691 defines the STM-64 gray optical interface. G.691 here is an expansion of G.957;
  • G.959.1 defines the specifications for inter-domain gray optical interfaces such as client-side signals 2.5G, 10G, 25G, 40G and PAM4 50G;
  • IEEE 802.3 defines interfaces at various Ethernet rates.

What are the characteristics of gray light? Let’s use an analogy. Gray light is like the crosshair of a shotgun. As long as it’s within this range, it can hit (transmit), but it won’t hit far. This is the same as gray light is suitable for short-range scenes.

In application scenarios, the light emitted by switches, routers, SDH networks, and WDM/OTN client-side optical modules mentioned below is all gray light. At the same time, gray light generally has a certain correlation with the light transmission window. For example, 850nm is generally short-distance, 1310nm is short-distance/medium-distance transmission, and the 1550nm window is generally used in long-distance and ultra-long-distance transmission.

There is another important scene for gray light , which is generally used when connecting with different manufacturers. This is also much easier than the colored light docking discussed below.

Next, let’s take a look at what colored light is?

Is colored light the same concept as the seven single-color lights of red, orange, yellow, green, cyan, indigo, and violet separated by natural light after passing through a prism ? These real colored lights in the rainbow are essentially mixed light, visible light, and are not the same thing as the colored light in communications, which refers to light of a single wavelength/frequency.

In order to meet the horizontal compatibility with the original transmission system interface, I think the concept of colored light has already appeared in ITU-T G.692, which specifies the frequency and wavelength of the nominal center frequency C/L band 80 channels (50GHz interval), It’s just that the speed is relatively small.

In the WDM system, each line-side optical module emits light that conforms to the wavelength and frequency specified in the standard protocol. The light here is what we call colored light. They combine light of different wavelengths in the same optical fiber through a multiplexer and splitter. medium transmission. It’s like dividing a large driveway into many small lanes, as shown in the picture below.

In the G.694x protocol, colored optical modules can be divided into (coarse wavelength division multiplexing) CWDM colored optical modules and (dense wavelength division multiplexing) DWDM colored optical modules according to different wavelength intervals. Among them, the wavelength interval of CWDM is 20nm, and the wavelength interval of DWDM is 0.8nm/100GHz (there are other intervals such as 0.4nm).

The DWDM color optical module is defined in the ITU-T G.694.1 standard, and the CWDM color optical module is defined by the ITU-T G.694.2 and G.695 standards. G.694.2 defines the center wavelength of CWDM, and G.695 defines its optical parameter values ​​and describes the optical physical layer, and how to convert non-standard wavelengths into colored light wavelengths (such as 1310nm gray light, through photoelectric-optical conversion It is a single wavelength colored light signal in CWDM).

The above standard protocols set the center frequency of colored light at one point, and the deviation of the center wavelength is relatively small. For example, the maximum center frequency deviation of DWDM is ±20GHz at an interval of 100GHz. This is like an AK with a sight installed (CWDM is equivalent to not having a sight installed).

So how to convert and connect between gray light and colored light?

Strictly speaking, it is impossible to directly connect the gray light module to the colored light module through optical fiber. Because one emits point light and the other emits astigmatism, the center frequencies are different, and of course there are limitations such as modulation packaging. Therefore, in the G.709 protocol, the encapsulation mapping scheme for converting client-side gray light to line-side color light in the OTN network is defined.

In terms of cost, generally speaking, colored light modules cost more than gray light modules, because gray light only requires the laser to emit light in a low-loss window. The colored light module needs to constrain the center frequency of the laser emission within a certain accuracy (maximum center frequency deviation), and also control the temperature (TEC) to ensure the stability of its wavelength. DWDM colored optical modules are more stringent than CWDM colored optical modules.

Of course, specific laser devices are involved here. For example, directly modulated DML lasers such as VCSEL/FB/DFB are cheap and low-cost (low speed and short distance), while indirect modulation EML lasers are high-cost (high speed and long distance). Therefore, how to choose gray light and colored light modules mainly depends on their specific applications.

For example, in the 5G prequel, we also have gray light solutions and color light solutions. Among them, CWDM6 (6 wavelengths), LWDM (12 wavelengths) and MWDM12 (12 wavelengths) can transmit multiple wavelength signals in one optical fiber and are colored optical solutions. Therefore, the colored light solution in 5G fronthaul can effectively save optical fiber resources, and is particularly suitable for scenarios where optical fiber resources are scarce.

Therefore, we usually choose gray light modules in scenarios with relatively short distances and abundant optical fiber resources. Colored optical modules are used in places where the optical fiber distance is far and the optical fiber is insufficient.

Any DWDM solution design and cost quotation, pls link with me, Taylor!!

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