what you need to known about single mode and multimode in fiber optics.

If you must know one thing about fiber-optic , it’s the difference between single-mode and multi-mode transceivers. Otherwise, you risk purchasing the wrong equipment or cable, or plugging incompatible cable into an optical device and compromising or destroying the signal.The single mode SFP or multimode SFP means the SFP transceivers which work at different types of optical fibers, ie single mode SFP will work with single mode fiber, while multimode SFP will work with multimode fiber.Therefore based on transmission distance to be covered as well as the overall budget allowed that Network installers will be able to detect the suitable transceiver mode for you. sometimes, installers run into challenges such as  devices you have in your network does not  correspond and work perfectly with the fiber. They plan to make a cable plant based on the multimode cabling, but owing to the link limitation or other reasons, they have to connect multimode equipment with single-mode devices. Is it feasible? Or put it more specifically, can I use the multimode SFP over single-mode fibers or vice versa? This article will give you a detailed illustration about the feasibility of the solutions, and introduce two relevant devices (mode conditioning cable and multimode to single-mode fiber media converter).

As we know, single-mode and multimode fibers are not interchangeable
the main reason is because single-mode fiber has much tighter tolerances for optics used. However, take Cisco SFP for example, there is a type of the GBIC SFP module (part number “SFP-GE-L” or “GLC-LH-SM“) which can support both single mode and multimode fiber. Single-mode fiber (MMF) uses a laser as a light source (the light beam is very concentrated), while multimode fiber (MMF) uses an LED to generate the signal. This would require two significantly different devices to generate the signal. Whether you’re considering single mode or multimode, it’s important to note that, although with similar form factors and optical connector interfaces (e.g. SFP+, QSFP+), different types of transceivers aren’t interchangeable, mainly due to differences in laser wavelength and fiber core size – and, more importantly, the designated speed and reach specifications.
Contrary to what you might think, Older multi-mode fiber used low-cost light emitting diodes (LEDs) to transmit their light signals.  One of the earliest multi-mode fibers, Optical Multi-mode 1 (OM1), has a larger core diameter of 62.5 microns.  Optical Multi-mode 2 (OM2) has a smaller core size of 50 microns and a higher bandwidth.  LED light sources cannot be modulated quickly, so in order to reach higher data rates, laser sources were required.  Optical Multi-mode 3 (OM3) was developed to work with vertical cavity surface emitting lasers (VCSELs) that operate at 750- 850nm.   Optical Multi-mode 4 (OM4) is also laser optimized but has a longer reach than OM3.  Single-mode fibers require longer wavelength, 1300nm or 1550nm laser sources.  These lasers are either Fabry-Perot (FP) or Distributed Feedback (DFB) type lasers.
When we choosing the SFP module, we must confirm the transmission distance and wavelength we want to use because the core sizes are completely  different between SMF and MMF. SMF is 9 micron and multimode is 62.5 or 50 micron. If users try to mix the single-mode and multimode cabling in the same network, they might have trouble dealing with the two different types of sign because the bandwidth (amount of information in the signal) of single-mode is higher than multi-mode. Um, much higher: as much as 100,000 GHz!. Then, in this situation, you can run successfully from a single-mode fiber transceiver over multimode fiber with the use of MCP cables, but the distance will not exceed the link specification for multimode transceivers. Otherwise, there will be much signal loss in the cable run.
In addition, this might be feasible but not advisable. If you need to make a connection between single-mode and multimode interfaces, you’d better use the intermediate switch that is able to convert the signals between single-mode and multimode fibers. The following part will introduce two solutions that might be helpful for the multimode and single-mode conversion.it is also possible to interconnect two devices using SMF interfaces at one end and MMF receiver at the other end. Keep in mind that it depends on the devices, so you can if you are lucky. When plugging LC single-mode duplex fibers on the multimode fiber transceiver (1000GBASE-SX) in the network, you will find the link came up (the light on the switch turns green). Therefore, the multimode fiber transceiver connected by the single-mode fibers works for short-reach application.
While it should be stressed that the link is not reliable and it only works for particular brand devices with a very short link length. Many sophisticated vendors like Huawei, Alcatel or Cisco do not support it. Nevertheless, owing to the differential mode delay (DMD) effect, signal loss of this connection is not acceptable in general. As noted before, mode conditioning cables, to some extent, can realize the connection between single-mode to multimode, but you can not say that you can convert single-mode to multimode or vice versa. Mode conversion between multimode and single-mode fibers often requires fiber to fiber media converters or the single-mode to multimode fiber converter.
As to the multimode fiber with single-mode SFPs, most people mention the mode conditioning patch (MCP) cables. The MCP cable is launched to support 1000BASE-LX optics over multimode cable plant. The mode conditioning cables allow customers to successfully run Gigabit Ethernet over our multimode cable using single-mode fiber transceivers, Cisco 1000BASE-LX/LH SFP is the special type of transceiver that can both support single-mode and multimode fibers. The image below displays the difference between standard SC multimode patch cable and SC mode conditioning patch cable.
NOTE
If you want to run multimode fiber optic cable over 1000BASE-LX SFPs, you can use the mode conditioning cable. However, mode conditioning patch cords are required for link distances greater than 984 feet (300 meters). For distance less than 300 m, please omit the mode conditioning patch cords (although there is no problem using it on short links).
Conclusion
It doesn’t really make much sense to use the single-mode fiber transceivers with multimode fibers in your network or vice versa, although the link will come up. Like I said above, you can if you are lucky connect. MCP cables and fiber to fiber converter are the two available options for single-mode and multimode connection. If you bought the wrong fiber optic cables, just replace it into the right one. Fiber optic cables and optical transceivers modules nowadays are very cheap. You won’t need to risk of mixing them in the same network.